Chapter 11 Goldfish and koi
This chapter covers those disorders likely to be seen in goldfish and koi, which constitute the most popular section of fish-keeping. Tolerance of wide temperature ranges means that these species can be kept outside, as well as inside, in most temperate countries such as Europe and North America. However, they are also happy at more tropical temperatures and in those countries such as Singapore, Malaysia and southern China where they are kept alongside ‘tropical species’. Hence, this disorders chapter should be read in conjunction with that on Tropical Freshwater Fish.
Fish-keeping is a huge worldwide hobby and industry. Unfortunately, veterinarians are often last to be consulted over a fish-related problem, or else they are approached purely as a source of antibiotics and other regulated medications. This is because:
|Temperature (°C)||10–30 (preferred range = 22–28 for Koi)|
|pH||6.0–8.4 (preferred 7.0–8.0)|
|Hardness (CaCO3) (mg/L)||100–250|
|Ammonia (total) (mg/L)||<0.02|
|Nitrate (above ambient tapwater levels) (mg/L)||<40|
Adapted from Jepson (2001).
If possible, fish should be examined in their home aquarium or pond. However, if the pond is large it may pay to ask for the fish to be caught and separated before arrival, as much time can be wasted attempting to catch the fish. Ponds are rarely built with recapture in mind. Once caught, place the fish on a damp towel for examination. If necessary sedate with MS222 or benzocaine (see ‘Anaesthesia’, below).
Always examine the ventral surface, as lesions here may not be obvious when viewed from above. Skin scrapes should be taken from the operculae, the flank, and around the base of the fins. Examine the gills and oral cavity.
This is best done under anaesthesia (see below). Blood can be drawn using a well-heparinized syringe from the ventral tail vein, which runs midline just below the caudal vertebrae. In small fish this can be accessed via the ventral midline while in larger fish, a lateral approach is often better. This same approach can be used for intravenous injections.
Provision of optimal water quality is essential to maximize recovery. A separate hospital aquarium or vat can be used but the water quality in this facility should be as good as in a main display (Fig. 11.2). The water should be filtered, but because of the use of medications such as antibiotics, biological filtration cannot be used. After each patient, the aquarium or vat should be dismantled and cleaned out with an iodine-based disinfectant.
Reliance is placed upon physical and chemical methods of water purification. Zeolite will absorb ammonia excreted by the fish, while activated charcoal will adsorb many harmful chemicals from the water. If salt is used, a small protein skimmer would be of great benefit. Ozonizers and ultraviolet sterilization are also useful adjuncts. Keep decorations to a minimum, giving just sufficient for nervous fish to hide behind. All materials used should be readily cleanable, such as plastic – and avoid live plants and bogwood where possible, as these can act as disease reservoirs. Temperature can be maintained at the optimum using commercial aquarium heaters. For koi and goldfish, a temperature of 18–25 °C should be considered. Keeping fish in a permanent 5 g/L solution of salt (use aquarium or sea salt, not table or rock salt) will reduce the osmotic load on a sick koi or goldfish.
The skin is a large and complex organ that is constantly in contact with the immediate environment of the fish. Its functions include an osmotic barrier, disease barrier, protection, intra-specific signaling and camouflage. Histologically it consists of:
Note that the scales of fish are dermal structures (not epidermal thickenings as seen in reptiles), therefore the loss of scale involves damage to the overlying epidermis and a potential breach in the fishes’ immune and osmotic barrier.