Chapter 12 Tropical freshwater fish
Around 80–90% of tropical freshwater fish are captive bred, with typical hot spots of production being Singapore (e.g. Fig. 12.1), Malaysia, Israel, Florida and the Czech and Slovak Republics. Some important fish to the industry are still wild-caught however, such as the cardinal tetra (Paracheirodon axelrodi) from the Amazon basin.
For general information on fish consultations, examination, nursing care and anaesthesia, see Chapter 11, Goldfish and Koi.
Recommended water quality parameters for different aquariums are listed in Table 12.1. Water hardness is measured in a variety of different ways with mg CaCO3 as international standard. Conversion factors from other units are given in Table 12.2. Common species of tropical freshwater fish presented to the veterinarian are listed in Table 12.3.
Table 12.1 Recommended parameters for: typical community aquarium; rift lake aquarium; discus aquarium
Table 12.2 Water hardness conversion factors
| Unit | Conversion factor to (mg CaCO3) |
|---|---|
| dH | 17.85 |
| Clark | 0.07 |
| f (French) | 0.1 |
| Hardness | 1 |
| Milliequivalent (mEq) | 0.02 |
Skin disorders
Structure and function of skin (see Ch. 11, Goldfish and Koi).
Differential diagnoses for skin disorders
Erosions and ulceration including fin rot
• Bacterial disease, typically Aeromonas spp, cytophagic-like bacteria, Flavobacterium spp. Also mycobacteriosis (see Musculoskeletal Disorders)
• Ulceration, haemorrhage, darkening of colour and exophthalmus in cichlids (Streptococcal infections)
• Flexibacter columnaris. Whitish erosive areas especially along back of fish, mouth fungus and shimmying (mollies) and tail rot (guppies). Sometimes known as false neon tetra disease when it occurs on small tetras
• Aphanomyces invadens mycosis (epizootic ulcerative syndrome, EUS). Susceptible species include striped snakehead (Channa striata), giant gourami (Osphronemus gouramy) and silver barb (Barbodes gonionotus) (see Miles et al 2001)
• Channel catfish virus disease is a herpes virus that affects fry and fingerling catfish. Infected fish swim erratically, are pale with multiple skin haemorrhages, severe exophthalmia and swollen abdomens
Nodules and non-healing wounds
• Hole-in-the-head disease (head and lateral line disease) (Spironucleus). Especially in discus, angelfish, oscars and other cichlids. Ulceration on the head and along lateral line (see also Gastrointestinal Tract Disorders)
• Wasting, darkening of skin colour and obvious boil-like swellings in the skin; in extreme cases it may have a sandpaper effect, due to the large number of granulomas present. Also exophthalmia, abnormal behaviour and swimming patterns (if the central nervous system is invaded) (Ichthyophonus hoferi). See also Neurological and Swimming Disorders, Ophthalmic Disorders and Systemic Disorders
• Gas-filled bubbles in the skin especially on the fins. Also occasionally behind the eye (gas bubble disease – usually secondary to oxygen supersaturation)
• Yellowish cystic lesions up to 5 mm diameter Clinostomum spp, a digenetic trematode (Wildgoose 1998).
Changes in pigmentation and colour
• Lethargy and reddened areas of skin. Fish stop feeding (bacterial septicaemia, Edwardsiella ictaruli in catfish)
• Obvious discrete white spots visible on skin and fins. Ichthyophthirius multifilis (white spot disease)
• Whitened areas of muscle, often accompanied by loss of colour, especially with neon tetras, other small tetras, killifish. Also may see emaciation and spinal curvature (Pleistophora – neon tetra disease). In white angelfish may cause localized accumulations of melanophores producing ‘black holes’
• Darkened colours, emaciation, abnormal swimming behaviour and mortalities in tilapia (Piscirickettsiosis-like organisms, PLOs). Some respiratory signs (gill hyperplasia)
• White tufts on surface that may resemble cotton wool (Epistylis, Saprolegnia, Flexibacter columnaris). Note that mucus strands hanging from the skin can strongly resemble Saprolegnia
• ‘Mouth fungus’ (along with fin-rot) is often described as a fungal infection, especially live-bearers such as mollies and guppies. This is usually due to cytophagic-like bacteria such as Flexibacter columnaris
• Greying skin (due to excessive mucus production). Ectoparasites – typically Trichodina, Chilonodella, Ichthyobodo, Gyrodactylus, Dactylogyrus
• Multiple small black spots in the skin and deeper in the muscles – so-called black spot disease. Digenetic trematodes, e.g. Neascus. The blackened cysts appear particularly obvious in light coloured fish such as silver dollars (Metynnus argentius)
• Darkening of body colouration, lethargy, loss of appetite, abdominal distension and mortalities in varieties of the three spot gourami (Trichogaster trichopterus). Also mortalities in the dwarf gourami (Colisia lalia) – an unspecified iridovirus
• Black spots – melanin concentrations – described in Mycobacterium marinum skin granulomas in Oreochromis mossambassicus (Noga et al 1990)
• Darkened colour, weight loss, excessive mucus production, rapid respiration, lethargy (‘discus plague’). Discus, occasionally angelfish (Pterophyllum species) (see Systemic Disorders)
• Algal dermatitis in farmed Metriaclima zebra has been described associated with deep invasion and persistent skin infections with Chlorochytrium and Scenedesmus spp of algae
Ectoparasites
• Protozoa
• Tetrahymena. Whitish areas or ‘spots’ often with ulceration. Commoner on live-bearers, cichlids (particularly dwarf cichlids) and tetras. Often associated with bacterial infections and white spot
• Icthyobodo necator (Costia necatrix). Common on catfish, killifish, anabantids, swordtails (Xiphophorus spp) and cichlids. See also Respiratory Tract Disorders
• Helminths
• Batracobdelloides tricarinata (Negm-Eldin & Davies 1999) (see also Cardiovascular and Haematological Disorders)
Neoplasia
• Lymphosarcoma. Start as nodules in the skin; eventually the overlying skin becomes devitalized and sloughs, leaving an ulcer. Internal lesions may also occur. Lymphosarcoma has been identified in pike (Esox spp) and Malawi cichlids (Metriaclima spp)
• Xiphophorus (swordtails and platies) hybrids are very prone to developing melanomas and neuroblastomas. Note that at least three viruses, a papovavirus and two retroviruses, have been implicated as predisposing factors
Other findings on clinical examination
• Breathing difficulties, irritation. Fins clamped. (Ichthyophthirius, Oodinium, Trichodina (see also Respiratory Tract Disorders)
Investigations
1 Skin scrape under light microscopy (see ‘Fig. 11.8’, in Goldfish and Koi)
a Protozoa
i Chilonodella is a large protozoan (30–70 μm) that has an almost oval, flattened appearance. Obvious cilia; moves with gliding, slow circular movement
ii Icthyobodo: even on high power, a small coma-shaped, very mobile parasite seen gyrating through the water
iii Oodinium: can be quite large, up to 1.0 mm diameter, oval shaped with a very dark appearance because of chloroplasts. Not usually mobile
Treatment/specific therapy
• Gas bubble disease
• Oxygen supersaturation usually secondary to pressurized oxygenation (waterfalls, malfunctioning venturi pumps, etc.), excess photosynthesis (high plant levels, suspended algae)
• Flexibacter columnaris
• Surfactants such as benzalkonium chloride are useful as a bath. Note that toxicity of benzalkonium chloride is increased in soft water so doses should be reduced if used in soft water (or if hardness not known) and the fish monitored closely for signs of distress
• Oodinium
• Antibiotic cover should be considered as secondary infections are common at the areas where the skin is damaged
• Tetrahymena
• Poor environmental conditions (high ammonia, high organic load, low water temperatures) predispose (Pimenta Leibowitz et al 2005)
• Try proprietary ectoparasitic medication. Can spread internally through the body musculature so treatment may not work
• Dactylogyrids are egg-layers; the egg stage is resistant to treatment and so infestations require multiple treatments up to 4 weeks apart depending upon water temperature
• Digenetic trematodes
• Complex life cycle involving a primary host such as a piscivorous bird, reptile or fish, and secondary snail hosts, so they rarely become a problem in aquaria
• Saprolegnia (fungal disease)
• Maintain the fish in a salt solution, as this will not only help to control the fungal infection, but will also help with the osmotic imbalance resulting from the infection. Salt solutions as low as 10 parts per thousand (ppt) (mg/100 mL) will inhibit Saprolegnial infections. Ideally aim for 1–3 g/L as a permanent solution until the problem has resolved. Care with corydoras catfish
• Anodonites trapesialis
• Larval stages described as parasitizing the epidermis of the cichlid Tilapia rendalli and the suckermouth catfish Hypostomus regani (Silva-Souza & Eiras 2002)
• Little inflammatory response but multiple microscopic skin lesions may predispose secondary skin infections
Table 12.4 Recommended dose rate for benzalkonium chloride
| Benzalkonium chloride concentration (mg/L) | Duration of bath (min) |
|---|---|
| 10 | 5–10 |
| 5 | 30 |
| 2 | 60 |
| 1 | Several hours |
Table 12.5 Recommended chloramine-T concentrations for different pH and water hardness combinations
| pH | Concentration in soft water (mg/L) | Concentration in hard water (mg/L) |
|---|---|---|
| 6.0 | 2.5 | 7.0 |
| 6.5 | 5.0 | 10.0 |
| 7.0 | 10.0 | 15.0 |
| 7.5 | 18.0 | 18.0 |
| 8.0 | 20.0 | 20.0 |
