Freshwater Temperate Fish

Most of the major taxonomic groups are represented by the freshwater temperate species.² For many, this group represents the species commonly considered as sport fish in the United States. The most common genera of freshwater temperate fishes maintained in captivity include the sturgeon (Acipenser spp.) (Figure 4-1), paddlefish (Polyodon spathula), eels (Anguilla spp.), pike (Esox spp.), bass (Micropterus spp.), sunfish (Lepomis spp.), walleye (Sander vitreus), mullet (Mugil spp.), spotted sea trout (Cynoscion spp.), salmonids, and cyprinids. Although many of these fish are raised by hobbyists, the majority of these species require much larger systems as might be represented in a public aquarium display. Consult an introductory ichthyology text for a more detailed description of the major taxonomic groups of freshwater temperate fish.

Figure 4-1 The sturgeon is a primitive freshwater temperate species. This genus has a wide range of species, representing relatively small sized animals to one of the largest freshwater species of fish. Although they are best known for their eggs (e.g., caviar), at least one species is offered for sale in the pet trade, and several others are routinely maintained in public aquaria.

Freshwater Tropical Fish

Freshwater tropical fish represent the largest numbers of animals typically found in home aquaria. Generally, this group of fishes is readily available for a small to moderate investment and can be easily maintained by the novice aquarist or hobbyist. Families of freshwater tropical fish include characins (e.g., tetras: Gymnocorymbus spp., Hemigrammus spp., Hyphessobrycon spp., Paracheirodon spp., Moenkhausia sp.), cyprinids (barbs: Barbus spp.), catfish (e.g., Corydoras spp., Pimelodus spp.), killifish (e.g., Aphyosemion spp., Fundulopanchax spp.), rainbowfishes (e.g., Iriatherina spp., Melanotaenia spp., Glossolepis sp.), gouramies (e.g., Colisa spp., Osphronemus spp., Trichogaster spp.), livebearers (e.g., Alfaro spp., Poecilia spp., Xiphophorus sp.), and cichlids (e.g., Pseudotropheus spp., Labidochromis spp., Iodotropheus sp., Dimidichromis sp., Copadichromis sp., Neolamprologus spp., Apistogramma spp., Microgeophagus sp., Aequidens spp., Cichlasoma spp.).^2,3

Catfish

Catfish (e.g., Ictalurus spp., Lacantunia spp., Corydoras spp., Ancistrus spp., Pimelodus spp., Arius spp., Kryptopterus spp., Phractocephalus spp.) represent one of the largest groups of freshwater fishes, with more than 2000 species. Catfish have a cosmopolitan distribution. Catfish are an important group because they serve many different roles, including as ornamentals (Figure 4-2, A), as food fish in aquaculture (Figure 4-2, B), as research animals, and for sport fishing. Most catfish are found in freshwater, although there are two families that contain saltwater species.^2,3 Although catfish have a cosmopolitan distribution, more than 50% of all catfish species are native to South America. There is a high degree of variability in the size and weight of these fish, with animals ranging from 10 cm to over 2 m in length and 10 g to over 300 kg in weight. Most species of catfish are nocturnal. Catfish are primarily benthic or bottom-dwellers. Because of their benthic lifestyle, catfish have sensory structures, barbels that assist them with characterizing food and nonfood items and substrate types in a low-light setting.

Figure 4-2 A, Corydoras sp. B, Ictalurus sp. Catfish represent a diverse group of fishes, with a significant amount of variability in morphology and physiology among genera. Although both are benthic, the catfish in A and B are shaped very differently.

Cichlids

The cichlids represent one of the most diverse groups of fish, with representation in North America, Central America, South America, and Africa. These fish are prized for their diversity in size, shape, and color. African lake cichlids are one of the most evolutionarily diverse groups in the world, as many different species have evolved within limited ecologic niches (Figure 4-3). With this rapid evolution have come highly variable morphology, feeding strategies, and dietary needs. Although cichlids can look highly variable from genus to genus, they all have a common “break” in their lateral line system. The lateral line is a mechanosensory structure that assists fish with interpreting events in their environment (e.g., shifts in water pressure suggesting a predator is approaching).

Figure 4-3 Cichlids represent one of the most popular groups of ornamental freshwater fishes kept in captivity.

Cyprinids

Goldfish (Carassius auratus), koi (Cyprinus carpio), and carp (Cyprinus spp.) are members of the largest family (>2200 species) of freshwater fish, Cyprinidae. Cyprinids have the widest area of distribution of any of the freshwater fish. Southeast Asia is the center of origin for this family of fish; however, many species have been introduced around the world and have readily adapted to these new environments.² The carp are the largest species from this family and may hybridize with goldfish. Koi (Figure 4-4) are colorful domestic mutations of carp that are highly valued by hobbyists and the commercial pet trade. The koi can be divided into two major classifications: the nonmetallic koi and the metallic koi.² This division is based on typical color patterns, scale types, shape, and size. Goldfish are not koi but rather descendants of the Crucian carp (Carassius carassius).² Goldfish remain one of the most popular ornamental species and are often found in garden ponds and home aquaria. These fish generally do best if kept in cool waters (55°–68° F, 12°–20° C) with abundant plant life and adequate aeration. There are more than 30 varieties of ornamental goldfish available in the pet trade today.

Figure 4-4 Koi (Cyprinus carpio) represent one of the most coveted groups of fish. These animals are prized for their color, size, and longevity. Their desirability is reflected in their value, with prized individuals selling for tens to hundreds of thousands of dollars.

Marine Tropical Fish

There are approximately 23 categories of marine tropical fish. These 23 groups can be arbitrarily divided into four major groups by their feeding attributes and compatibility with other species (Figure 4-5). Most of these groups are well represented in the aquarium trade and in public aquaria.

Figure 4-5 Marine tropicals should be housed based on their feeding strategy. This figure represents the typical setting for “rapid eaters.”

The first category of marine tropical fish is the “rapid eater” and includes the angelfish (e.g., Pomacanthus spp.), damselfish (e.g., Amblyglyphidodon spp.), squirrelfish (e.g., Holocentrus spp.), triggerfish (e.g., Balistapus spp., Balistoides spp.), and groupers (e.g., Cephalopholis spp., Variola spp.).² These fish do well if kept at low densities in the aquarium or if they are provided a large area where overcrowding is not an issue (e.g., large reef tanks in public displays). Hostile interaction is a major concern for these animals, as they can be very food aggressive.^2–4 These animals may be kept together quite readily if provided an abundance of food and a diverse diet. Often they are kept in live coral reef tanks with anemones and other species of animals from the same group.^2,3

The slow eaters represent the largest subgroup of marine tropical fish. This group includes, but is not limited to, the anemone clown fish (e.g., Amphiprion ocellaris), parrotfish (e.g., Sparisoma spp.), puffers (e.g., Carinotetraodon spp., Tetraodon spp., Colomesus spp.), surgeonfish (e.g., Acanthurus sohal), wrasses (e.g., Cheilinus spp., Halichoeres spp., Hemigymnus spp., Thalassoma spp.), and trunkfish (e.g., Aulostomus spp., Strophiurichthys spp., Ostracion spp.). Compatibility is an issue with this group of animals, and they do best if maintained in large displays with a large amount of hiding area. Although these fish are grouped based on their feeding strategy, there remains a great deal of variability in the diets of these animals.

Another group extreme in feeding habits includes those animals that have difficulty competing for food. This group of animals includes some of the more unusual species, such as the seahorses (Hippocampus spp.), jawfishes (Opistognathus spp., Stalix spp., Lonchopisthus spp.), pipefish (Stigmatopora spp., Lissocampus spp., Corythoichthys spp.), and batfish (Dibranchus spp., Halieutea spp., Halieutichthys spp., Malthopsis spp., Ogcocephalus spp., Zalieutes spp.). Many of these fish are slow swimming and are easily outcompeted by faster swimming fish. Members of this group should be housed together with similar species and monitored closely to ensure that they obtain sufficient calories.

The last group of animals to be categorized as marine tropicals includes the snappers (e.g., Lutjanus spp., Pristipomoides spp., Nemadactylus spp., Etelis spp.) and grunts (e.g., Haemulon spp.). These are reasonably large, territorial schooling fish that are, for the most part, kept at public facilities. They can best be described as “gluttons,” not only for the manner in which they feed but also for the almost insatiable hunger they exhibit. They are virtually fearless in their feeding habits and will attempt to remove food from the jaws of even large predators.²

Marine Coldwater Fishes

Although some of the smaller marine coldwater species are used for public display, many are too large to be accommodated in anything other than large aquaria. The majority of the species in this general category are used as commercial food fish. The first group, which represents approximately 70 different species, includes the gadids or cod (e.g., Gadus spp.), haddock (e.g., Melanogrammus spp.), hake (e.g., Urophycis spp.), and pollock (e.g., Theragra spp.). These species are valued as food and for their medicinal value. There are very few public aquaria that exhibit these large species, because they require systems in excess of 1 million gallons of seawater. Tuna (e.g., Thunnus spp., Euthynnus spp., Katsuwonus spp.) have become popular in large open ocean exhibits at public facilities, but only through improved and advanced life support technology and husbandry techniques has it been possible to exhibit this spectacular group of animals. By far the most common category of fish in this group is the flatfish. This group includes, but is not limited to, the flounder (e.g., Platichthys spp., Scophthalmus spp., Limanda spp., Pleuronectes spp., Atheresthes spp.), halibut (e.g., Hippoglossus spp.), and rock sole (Lepidopsetta bilineata). The sablefish (Anaplopoma fimbria) and lumpfish (Paraliparis fimbriatus) are also in this group of fish and are only occasionally represented in public displays.

Elasmobranchs

There are more than 350 species of sharks in the world, and only a small percentage are represented as display animals or kept by private aquarists and hobbyists. The species that are found occasionally in home aquaria include the carpet sharks (e.g., Parascyllium spp.) (Figure 4-6), catsharks (e.g., Galeus spp., Scyliorhinus spp.), and horned sharks (e.g., Heterodontus spp.). Most of the other species of sharks are much too large to be kept in anything less than a public aquarium or large commercial facility. Many public facilities display saw sharks (e.g., Pristiophorus spp.) and multiple species of ornamental sharks, including angel sharks (e.g., Squatina australis), dogfish (e.g., Squalus spp., Scymnodon spp., Deania spp., Centroscymnus spp.), and frilled sharks (e.g., Chlamydoselachus spp.).

Figure 4-6 The carpet shark (Parascyllium collare) is one of the few species of elasmobranchs that can be kept in captivity by hobbyists.

Skates and rays are closely related to sharks; however, they are markedly different in appearance (Figure 4-7). There are both freshwater and saltwater varieties of stingrays. The skates and rays are dorsoventrally flattened animals and usually have at least one venomous spine on the dorsal caudal fin (tail). There are multiple freshwater species (Figure 4-8) that are small enough to be kept by the private aquarist; however, the majority of these animals are also found in public facilities. There are a number of ornamental species of saltwater rays, such as the guitarfish (e.g., Rhinobatos spp.), butterfly rays (e.g., Gymnura spp.), and cow-nose rays (e.g., Rhinoptera spp.), that have been maintained and successfully reproduced in public facilities. Many of the larger, saltwater rays, such as the southern stingray (Dasyatis americana), Atlantic stingray (Dasyatis sabina), eagle ray (Myliobatis aquila), and giant manta ray (Manta birostris), are also now being kept in public aquaria.

Figure 4-7 Sharks and stingrays have distinct morphologic features. Note the dorsoventrally flattened shape of the benthic (bottom-dwelling) ray compared to the streamlined pelagic (open water–dwelling) shark.

Figure 4-8 Freshwater stingrays are routinely found for sale in the pet trade. These animals originate from South America.

Environmental Considerations

It is easy to recognize aquatic facilities (institutional or pet retail) that have the most disease problems on the basis of their appearance. There is usually an accumulation of trash, dirty exhibits, dead animals, and a general “unhealthy” appearance to the collection. This does not mean that a “clean” facility is free of disease but that they traditionally have fewer and less severe outbreaks of disease.⁵ The general husbandry and maintenance of the aquatic facility and ecosystems have a direct relationship to the overall health of the animals. Maintaining clean areas behind the exhibits and nonpublic areas is essential to good health practices.^3,5 The same can be said for retail aquarium facilities. Accumulation of feces, excess food, and detritus are predisposing factors to poor water quality and can serve as substrates for facultative pathogens. Cleaning and disinfecting seines, nets, buckets, and tanks are imperative to maintain a high-quality health program at a facility. A dynamic team effort is required to maintain a clean facility and healthy animals, but a clean facility will pay dividends by providing a safer workplace, reduced disease incidence, increased production, and generally healthier fish.

Given ideal environmental conditions, ecosystems require a certain amount of space to fully develop, though quantitatively how much space is a debatable matter. Generally, to place an ecosystem in a very large aquarium or other holding space is not a major ecologic problem, although it might be considered an engineering endeavor. The difficulty arises when veterinarians attempt to scale down and include many components in a much smaller space than which would normally occur in the wild. To miniaturize an ecosystem and place it into a small space for observation, education, or research, veterinarians are immediately faced with a major dilemma, which is to scale the miniature so that it can still function as a reasonable facsimile of the wild ecosystem. This question is intimately related to the entire problem of how veterinarians affect wild environments and how they restore them, as well as how they construct their aquaria.^5,6

Creating an Aquarium

There is little biologic reason for the traditional “box type” aquarium shape; however, the common reason for its existence is associated with availability and mechanical, aesthetic, and economic convenience (Figure 4-11). For many scientists and aquarists, the ease of purchase and setup of a ready-made tank outweighs all other factors when a water-based system is desired.

Figure 4-11 Historically, all fish tanks were rectangular (A). More recently, there has been a movement by commercial tank manufacturers to create new tank shapes (e.g., bow front tank) (B). These oddly shaped tanks do little for the aquatic ecosystem, and are primarily for aesthetics.

The presence of tank walls that can support benthic communities or allow excessive light is undesirable. A weakly translucent cylindrical tank that minimizes attachment surface for a given volume and has a rotating, cleaning mechanism to keep the surface free of sediment is highly desirable but very expensive.

For the hobbyist, aquarist, or scientist who wishes to construct an efficient model ecosystem, the materials (e.g., glass, plexiglass) are readily available to fabricate any shape or form of enclosure. Only after determining the ideal shape of the desired system, should one be concerned with the aesthetics, viewing, and construction of the aquarium.

FILTRATION

In nature, the waste products produced by fish are diluted into the vastness of the body of water and carried away by flowing water, reducing the potential dangers to fish. In closed systems (e.g., home or public aquarium), wastes and toxins can accumulate to levels that are harmful to fish. To avoid this problem, closed systems must be managed using some form of filtration. There are three primary types of filtration: mechanical, biologic, and chemical. These different filtration mechanisms work independently of one another and can be used in combination.

Mechanical Filtration

Mechanical filtration represents one of the original forms of filtration used in the home aquarium. This type of filtration removes organic debris from the water by passing it through a filter material (e.g., floss, fiber, or paper cartridge) (Figure 4-12). The amount of filtration that can be accomplished using this type of filter depends on the type and size of the filter material and rate that the water is recirculated through the filter media. A densely packed fiber or small pore size will restrict the size of waste that can pass the filter media, resulting in less waste in the aquarium. Maintenance of these filters requires cleaning or replacement of the floss or cartridge. Mechanical filtration remains an important method of filtration in home aquaria, outdoor ponds, and public aquaria. This type of filtration is best used with other types of filtration (biologic or chemical) to improve the overall quality of water in the system. When a series of filters is used inline, it is best to place the mechanical filter first. This ensures that the heavy organic material will be removed before contaminating or clogging the other filters (e.g., chemical or biologic-sand filter). Mechanical filtration does have limitations; for example, it is not effective in trapping finite particles or chemicals.

Figure 4-12 Mechanical filtration is used to remove detritus from an aquatic system. This type of filtration can be combined with the other methods to improve the overall water quality in a system. For example, this power filter cartridge is comprised of floss (mechanical) and carbon (chemical). Once in the system, it will also be colonized with nitrifying bacteria (biologic).

Biologic Filtration

Ammonia is the primary end product of protein catabolism in fish. In a closed system, this waste product can be fatal to fish. It was the advent of biologic filtration that led to our ability to maintain large densities of fish in small volumes of water. Biologic filtration is the most common type of filtration used in the home aquarium and outdoor pond, and it comes in many different forms (e.g., under-gravel filters, bio-wheels, sand or bead filters, and wet-dry filters) (Figure 4-13). A biologic filter should be selected based upon the expected load on the system. If a large density of fish is going to be maintained in a system or the fish are going to be fed large quantities of food to ensure growth (e.g., aquaculture), then a large surface area for bacteria is needed.

Figure 4-13 Biologic filtration is essential to maintaining fish in closed aquatic systems. There are many different ways to increase the overall surface area in a system to colonize nitrifying bacteria. This filtration method uses plastic balls as a surface for the bacteria.

The biologic filter is comprised of nitrifying bacteria. Although there are numerous types, the two most common genera discussed are Nitrosomonas spp. and Nitrobacter spp. Nitrosomonas spp. are important in denaturing ammonia, the primary waste product produced by fish, into nitrite. Both ammonia and nitrite are toxic to fish. Nitrobacter spp. are responsible for further reducing nitrite to nitrate.

Once an aquarium and biologic filter are colonized, the system becomes self-sufficient. However, there are several factors that can affect the function of a biologic filter, including temperature, oxygen content, and drugs/therapeutics. Nitrobacter spp. are not cold tolerant. Therefore, in outdoor ponds when the water temperature drops below 65° F (18.5° C), nitrite will not be converted into nitrate as rapidly. During those times when the water temperature may drop below 65° F, fish should be fed less food to reduce the load on the system. Nitrosomonas spp. and Nitrobacter spp. are aerobic bacteria. In the well-aerated home aquarium, oxygen levels are often adequate for the bacteria; however, in outdoor ponds, oxygen levels can become depleted on warm summer nights. To reduce the likelihood of biologic filter failure, it is recommended that outdoor ponds be aerated during warm, summer days and nights.

A biologic filter requires time to become established. The amount of time depends upon the temperature of the water and the organic load on the system. New systems should be started slowly, adding only a small number of fish at a time. Closely monitoring the ammonia, nitrite, and nitrate levels on a daily basis is strongly recommended for new systems. Commercial microbial products (Fritzyme; Fritz Industries, Dallas, TX) are available that can expedite the establishment of the filter by seeding the system with bacteria. Water samples, filter pads, or aquarium substrates from established systems have also been used to seed a tank. However, the addition of these products to a new system may lead to the introduction of potential fish pathogens.

Chemical Filtration

There are numerous types of chemical filters in the pet trade. Chemical filtration refers to those filters that remove toxic compounds by binding them or converting them into nontoxic substances. The original form of chemical filtration was activated carbon. Carbon serves as a nonspecific binding agent for a number of different substances. When the binding sites on the carbon are full, they no longer act as filters and need to be replaced or cleaned. Other forms of chemical filtration are more specific, such as the resins that only bind ammonia. There are other forms of chemical filtration, such as ultraviolet (UV) sterilizers and protein skimmers, that alter or trap compounds. UV sterilizers expose compounds to short wavelength light, altering their form and rendering them harmless. UV sterilizers can be used to control certain pathogens and algae. A UV sterilizer has a UV bulb encased in a waterproof sheath within a cylinder. As water passes through the cylinder, the water is exposed to UV light, which can alter the DNA or RNA of the organism. The amount of time that it takes for the water to pass the bulb and the bulb wattage determine the effectiveness of the UV sterilizer. A low-wattage bulb in a short cylinder will have little effect on pathogens. These systems have also been used with great success at controlling algae and pathogens in outdoor ponds. Protein skimmers trap proteins in bubbles so that they can be separated from the water and removed. Chemical filtration, in combination with mechanical or biologic filtration, can improve the water quality dramatically, creating a “healthy” environment for fish.

WATER QUALITY ANALYSIS

The aquatic environment is a very complex system that is subject to constant physical and chemical changes. A water-assessment program should be devised for monitoring an aquatic ecosystem. Many factors need to be taken into consideration when designing a water quality monitoring program, including the volume of water in the aquarium/aquaria, the number and type of animals that will be present, the type of life support system, and the period of time the aquarium has been set up. Ideally, a water-assessment program should be established before designing and setting up a system, keeping in mind that most of the water quality analysis is done to keep its inhabitants healthy.⁸

Water testing should be done daily for new aquaria until the system has cycled. When collecting a water sample, it is important to collect a mid-water sample, as samples closer to the surface or substrate will reflect more extreme values.^2,3 Portable water analysis kits use premeasured reagents and simple analytic methods that may compromise the degree of accuracy. However, they are suitable for most private aquarists and small backyard ponds. Most large, public aquaria use more advanced analytic methods, such as mass spectrometry, saturometers, and sophisticated chemical analysis, for evaluating water quality. The acquisition and utilization of many of these processes and techniques are beyond the scope of what most private aquarists can afford. Commercial water quality laboratories can perform a complete water analysis for a modest price, and it may be a good idea to submit a sample to a laboratory for a baseline measurement. This method also offers the added benefit of comparing standardized values with the parameters derived from veterinarians’ own testing methods.

Water quality is very important to the health of a fish, and poor water quality can prove fatal. There are two types of systems that can be used: open and closed. In open water systems, the water in the aquarium is continually replenished using a fresh water source. An individual who lives near the ocean may collect seawater for a home aquarium, although that is not recommended because of potential contaminants in the water. Open water systems are rarely used because they are labor intensive and require regular exchange of the entire system. The majority of home aquaria utilize closed recirculating systems, which recirculate the same water over and over again using a filter. In the closed system, fresh water is added only after evaporation or at the time of a water change.

Ammonia, Nitrite, and Nitrate

Ammonia is produced in fish as an end product of protein catabolism. This waste product is primarily excreted via the gills, although some excretion in the feces also occurs. Ammonia is also generated in the aquatic system from the breakdown of uneaten food and detritus. Ammonia nitrogen can occur in two forms: ammonium (NH₄⁺) and ammonia (NH₃). Ammonia is the more toxic form for fish.¹¹ The relative concentration of each form varies with pH and water temperature.¹² Ammonia is soluble in water, and minimal amounts are lost through evaporation. In a closed system, such as an aquarium or backyard pond, ammonia levels can build up to toxic quantities (>1 ppm). Even low levels of ammonia can be toxic to the gills and skin, resulting in increased susceptibility to infections. Fish suffering from ammonia toxicity appear irritated, gasp at the water’s surface, and rub against rocks in the aquarium as a result of the irritation caused by the toxin. Ammonia levels should be monitored closely in closed systems, especially those with high fish densities. Testing for ammonia should be done weekly using a standardized commercial test kit, which is available at local pet retailers. In an established system, ammonia levels should be zero. If the ammonia levels begin to rise, then the system should be reevaluated. Overfeeding and overstocking an aquarium can overburden the biologic filter. Severe temperature fluctuations and insufficient oxygen levels may also result in a significant loss of the biologic filter. This is especially common in ponds that have significant summer algal blooms. New systems require time to become established, and new fish should be added gradually to prevent an overload of the biologic filter.

Because ammonia is a common waste product in the aquarium, most problems can be prevented by limiting the numbers of fish in the aquarium and regulating the amount of feed being offered. A general rule of thumb for feeding fish is to only offer what they can eat in a 2- to 5-minute period. In cases where ammonia levels are creating problems, the first recommendation is to remove 25% to 50% of the water from the system and replace it with fresh, dechlorinated water. There are commercial products available that can chelate the ammonia source, but they are only a temporary solution. The primary cause of the elevated ammonia levels must be diagnosed and corrected.

Ammonia is a colorless, odorless substance that can cause significant mortalities in a home aquarium. Most inex-perienced aquarists tend to single out infectious diseases when they experience fish losses; however, poor water quality (e.g., excessive ammonia or nitrite) is often a primary/secondary cause of mortalities in these animals and should be tested on a regular basis.

The nitrogen cycle eliminates ammonia by converting it to less toxic compounds. The first step in the cycle is to convert ammonia to nitrite. Nitrosomonas spp. are the primary bacteria associated with this process. Unfortunately, nitrite is also toxic (>0.1 ppm) to fish and can be rapidly absorbed across the gills. Affected animals may develop a methemoglobinemia and have a characteristic “brown blood.” This blood dyscrasia leads to reduced erythrocyte oxygenation and respiratory compromise. Fish with nitrite toxicity may behave similarly to fish with ammonia toxicity, and be found gasping for air at the water surface and die suddenly. When fish show clinical signs associated with nitrite toxicity, they should be removed from the toxic water and placed into a fresh, dechlorinated, well-oxygenated system. A significant water change (25%-50%) should be made in the original aquarium or pond and the biologic filter reestablished. Salt can also be used to diminish the toxicity associated with salt.

The second step of the nitrogen cycle occurs when Nitrobacter spp. oxidizes nitrite to nitrate. Nitrate levels less than 0.5 ppm are generally regarded as safe; levels less than 5.0 ppm are associated with stress and may predispose fish to opportunistic infections; levels greater than 10 ppm are considered toxic for some species. Reports of nitrate toxicity are rare in freshwater and saltwater fish, but at elevated levels they may be stressful and predispose the animals to opportunistic pathogens. Nitrate is utilized by plants and algae as a food source. Nitrate can be removed from an aquatic system by performing regular water changes.

Ammonia and nitrite levels in an aquatic system may rise soon after treatment of the water with antibacterial compounds or a reduction in water temperature. Antibiotics added to the water are nonselective and may kill both pathogenic and commensal organisms. If these compounds kill enough of the bacteria associated with the biologic filter, then oxidation of ammonia and nitrite may stop. Nitrosomonas spp. are more temperature tolerant and will recolonize before Nitrobacter spp., so ammonia levels should be expected to decrease before nitrites. Therefore, elevated nitrite levels are often detected soon after a reduction in water temperature.

“New Tank” Syndrome

“New tank” syndrome is a common occurrence with beginner aquarists and primarily occurs when fish are overstocked in a new aquarium. If large numbers of fish are added to an aquarium, and the biologic filter is not established, the system will be unable to eliminate the ammonia produced by the fish. In most cases, the owners report an acute mortality event with clinical signs consistent with ammonia and nitrite toxicity. These problems can be prevented if the new owner is patient and realizes the importance of providing a break-in for the filter (4-6 weeks). Fish should be stocked gradually, usually one to two fish per week. A standard rule of thumb for a freshwater stocking density is 1 to 1.5 inches of fish per gallon of water; in saltwater systems, the stocking density should be 2 to 2.5 inches of fish per gallon of water. With the advent of new filtration systems, stocking densities will continue to increase; however, if the filter becomes compromised or fails, the results would be disastrous.

Oxygen

In the aquatic system, oxygen diffuses into water at the surface when the surface tension of the water is broken. For home aquaria, this occurs regularly when external filters are used that “drop” the water back into the tank like a waterfall or when airstones are used. The amount of available or dissolved oxygen (DO) within the system can be measured using special equipment. In most cases, a DO greater than 5 ppm is sufficient to maintain fish. For the home hobbyist, oxygen depletion is generally only a major problem with outdoor ponds during the summer months. During the day, plants produce their own food (photosynthesis) by removing carbon dioxide from the water and using energy produced by the sun. As plants make their food, they release oxygen into the water. During the night when plants or algae cannot undergo photosynthesis, they actually consume oxygen. In ponds with a large number of plants or algae, the oxygen levels in the water can fall to dangerously low levels (<3 ppm) for the fish. Another factor that may affect oxygen levels in the water column is temperature. Oxygen is lost to the atmosphere more rapidly in warm water than cold water. The use of aerators or fountains, especially at night, will help maintain adequate levels of oxygen in a pond.

Water pH

The pH of water is measured by taking the negative logarithm of hydrogen ions in the water. In simplest terms, pH can de divided into three categories: acid, neutral, and basic. The range of pH values fits on a scale of 1 to 14. Values below 7.0 represent acidic water, values between 7.0 and 7.9 are neutral, and values above 8.0 are basic. The pH in most aquaria and ponds should fall between 6.5 and 8.5. In the extreme ranges (<4.0 or >10.0), water would be so acidic or basic that it would burn the fish. This means that the difference between 7.0 and 8.0 is much more significant than expected, because the pH values are based on a logarithmic scale. Therefore, if the pH is allowed to fluctuate, fish will become stressed and more susceptible to disease. The pH of natural bodies of water varies based on the substrate, water shed, and other environmental factors. Fish from Central America and South America thrive in water that is neutral or slightly acidic, whereas fish from Africa and Asia thrive better at neutral to alkaline water.

Water should always be tested before replacement into an aquarium. There are a number of factors that may affect the pH in an aquarium or pond, including the biologic filter, fish density, vegetation, and algae. Biologic filtration actually produces acid when ammonia is converted into nitrite. If the water has a low buffering capacity, and the aquarium or pond biologic filter is converting a large amount of ammonia, the pH could become very acidic. Fish produce carbon dioxide (CO₂) as a waste product. When an aquarium or pond is heavily stocked with fish, the amount of CO₂ can build up in the water. Carbon dioxide promotes acid production and can actually decrease the pH (acidic). Plants and algae, which use photosynthesis during the day to make energy, utilize CO₂. However, at night, plants and algae utilize oxygen (like fish) and expel CO₂ as a waste product. In a system with a large number of plants and a large amount of algae, the pH can drop to a dangerously low level. Fish die-offs in ponds are often associated with high CO₂, low pH, and low oxygen levels. When plants or fish die, they also release compounds that can lower the pH. To prevent this from becoming a problem, always immediately remove dead fish or plants.

Chlorine

Chlorine is an elemental gas that is added to municipal water as a disinfectant. Unfortunately, chlorine is also toxic to fish. Fish that are exposed to chlorinated water can develop life-threatening respiratory distress. Chlorine readily crosses the gills of fish and blocks the animal’s ability to absorb oxygen. Fortunately, chlorine can be removed from the water by allowing it to de-gas for 48 to 72 hours or by adding a dechlorinator (e.g., sodium thiosulfate). Chlorine levels in municipal water can fluctuate with the season; thus, questions about the timing or the amounts of chlorine that are being added in a specific municipality should be directed to the local water company. Chlorine levels in a water sample can be tested using a commercial test kit purchased from a local pet retailer.

Chloramine

Chloramine represents another disinfectant that is routinely added to municipal water for sterilization purposes. Chloramines are a combination of chlorine and ammonia. Commercial dechlorinators can still be used to remove the chlorine from this molecule, but they do nothing to the ammonia. Because ammonia is toxic to fish, it is important that levels of this compound do not reach levels greater than 1 ppm. Fortunately, a functional biologic filter will prevent this from occurring.

Hardness

Hardness measures the quantity of divalent cations (e.g., calcium and magnesium) in the water. In natural waters, the divalent ions are derived from the limestone, salts, and soils. The general range for hardness in freshwater systems is 0 to 250 mg/L, whereas in saltwater systems, hardness can exceed 10,000 mg/L.¹³ The divalent cations that represent hardness, calcium and magnesium, play an intricate role in water quality conditions. Calcium can alter osmoregulatory function in fish during times of low pH or elevated ammonia.¹³ Calcium and magnesium are both important for growth and development in fish fry.¹⁴ These minerals can also protect fish against heavy metals by competing for gill absorption sites. Copper is routinely used to treat parasites. Calcium and magnesium compete with the copper for absorption sites, reducing the effectiveness of the copper. Distilled water should never be used to replenish water in an aquarium because it is deficient in these essential cations.

Alkalinity

In natural aquatic systems (e.g., lakes, ponds, rivers, streams), fish are exposed to a relatively stable pH because of buffers. The most common buffers in aquatic systems are bicarbonate (HCO₃) and carbonate (CO₃²⁻). Other buffers that may occur in water in lesser amounts include hydroxide (OH⁻), silicates, phosphates, and borates. The quantity of buffers within a system depends upon the source of the water. Some municipal water supplies contain relatively low concentrations of buffers, whereas others may have large concentrations. In cases where the water has a low buffering capacity, commercial buffers can be purchased from a local pet store and added to an aquarium to create a more stable pH.

Total alkalinity can also play a protective role against potential heavy metal toxins. Heavy metals (e.g., copper and lead) in the water can be absorbed at the level of the gills, build up to toxic levels, and lead to the eventual death of an affected fish. Bicarbonate and carbonate can protect against heavy metals by chelating them in the water, rendering them harmless. This is important to remember when treating fish with nonchelated copper. If the alkalinity is high, the nonchelated copper may be bound and rendered useless.

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