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Diagnosing common fish diseases

When fish get sick, the actual number of possible explanations is relatively small. By far the most common causes of ill health are water quality and water chemistry problems, which typically manifest themselves in issues such as finrot and fungus. The next most common problem is whitespot, a highly contagious disease almost always brought into an established aquarium with new livestock that hasn’t been properly quarantined.

Some common problems tend to be specific to certain types of fish. These include hole-in-the-head disease (which affects mostly cichlids); neon tetra disease (neons and other small tetras); dwarf gourami iridovirus (just dwarf gouramis); and something called the shimmies (mostly mollies, but occasionally other livebearers too).

In this article we’ll sort through the most common fish diseases, first identifying their symptoms, and then looking at what needs to be done to cure affected fish. It’s worth mentioning that almost all these diseases are easy to prevent, so understanding what causes them is especially important.

Damage to the skin, mouth and fins (finrot, fungus and mouth fungus)

Finrot is one of the easiest diseases to diagnose. The fins become eroded, meaning that the membrane disappears, often leaving behind some of the rays, so the fin has a ragged edge. Despite the name, finrot can develop on other parts of the fish, not just the fins (though the fins are usually the part of the fish where the infection begins). Finrot on the body typically looks like a bloody patch or sore. Dead white skin often the surrounds the raw, bloody tissue underneath.

Finrot is a bacterial infection that occurs for one of two main reasons. The first is chronically poor environmental conditions. This includes things like poor water quality, the wrong water chemistry, and water that is too cold for the species being maintained. Without treatment, finrot can develop into septicaemia and kill the fish. Finrot is best treated with antibacterial medications available from aquarium shops. Severe cases may require antibiotics that require a prescription from a vet.

Fungus and mouth fungus are two similar, but distinct, diseases. Fungal infections have a fluffy appearance like white cotton wool. Mouth fungus, despite the name, is a bacterial infection. Typically mouth fungus looks like white slime around the mouth, sometimes fluffy-looking, hence the comparison with fungal infections. Fungal infections frequently follow on when fish are physically damaged, but they can also occur for the same reasons as finrot. Mouth fungus is very like finrot in being symptomatic of chronically poor environmental conditions. Both can kill fish if not treated.

Fungus is best treated with proprietary medications, though aquarium salt at 1-2 grammes per litre can help with light cases or be used as a preventative in situations where fish are damaged and fungus is a possibility. At 10 grammes per litre aquarium salt is an effective fungicide, but this salinity is only viable when salt tolerant fish are being treated, such as mollies and guppies. Mouth fungus, being a bacterial infection, requires similar treatment to finrot.

Damage to the eyes (exophthalmia)

Exophthalmia, sometimes called pop-eye, is the most common eye problem. One or both eyes become cloudy and expand, pushing outwards from the skull. Exophthalmia is caused by a bacterial infection, though the reasons for this vary. If just one eye is affected, physical damage is often the cause, such as the fish bumping into the side of the tank. When both eyes are affected, the problem is usually chronically poor environmental conditions.

Without treatment, the affected eye usually ends up falling out, resulting in blindness. The best treatment is the antibiotic nitrofurazone, a prescription-only drug available from vets. Needless to say, conditions should also be improved. If fish are damaging themselves, the tank may be too small, or there may be aggression between the fish. Chronically poor water conditions should be remedied as well.

Tiny white specks on the fins and body (whitespot and velvet)

There are two common diseases that manifest themselves as white grains on the fins and body: whitespot and velvet. The best way to tell them apart is by the size of the grains. Whitespot cysts are larger, more like salt grains, while velvet cysts are much finer, like icing sugar. Velvet also tends to produce a distinctive golden sheen on the fish, hence the name. Whitespot is probably the more common of the two diseases. Both are highly contagious and if not treated can allow secondary infections to set in, killing the fish.

Both whitespot and velvet are easy to prevent. Since newly purchased fish frequently carry these parasites, quarantining such fish for 2-3 weeks before putting them in a community tank is highly recommended. To treat infections, either use a proprietary whitespot and velvet medication, or else use aquarium salt. If using salt, start by raising the temperature to 28-30 degrees C. This speeds up the life cycle of the parasite. Add salt at a dose of 2 grammes per litre, and leave the tank running this way for 2 weeks. Although proprietary medications are widely sold, they can be toxic to sensitive fish like stingrays, loaches, mormyrids and some catfish. Using salt is generally kinder to the fish and less stressful to these sensitive fish.

Excessive slime production (slime disease)

Slime disease, commonly referred to by an old name for the parasite involved, Costia, causes the infected fish to produce lots of slime, resulting in grey patches on the body. Infected fish also tend to have trouble breathing, so may be moving their gills unusually quickly.

Without treatment the infected fish will quickly weaken and die. Slime disease may be treated with proprietary medications or through the use of aquarium salt. If using salt, add 3 to 5 grammes per litre, and leave the aquarium running this way for two weeks at least. Infected fish should also be dipped into a more concentrated salt solution at 20-35 grammes per litre for 2-20 minutes, depending on the size and salt tolerance of the fish. Most freshwater fish find saltwater dips stressful, and if the fish shows signs of severe stress (such as loss of balance) it should be removed and returned to the aquarium.

Off-white, textured growths on the fins and body (lymphocystis)

Lymphocystis is a viral disease that causes odd growths on the fish often likened in texture to cauliflower. The colour of these growths varies from off-white to pale coffee colour. Lymphocystis is unsightly rather than dangerous, unless the growths obstruct an orifice. Lymphocystis is likely caused by environmental stress, and given good conditions, clears up by itself. Besides good water quality and a healthy diet, nothing else is required. Relatively few freshwater fish are affected by lymphocystis, typically advanced perciform fish such as cichlids, glassfish, scats and spiny eels. It’s unlikely to be seen among barbs, tetras, catfish and other non-perciform fish.

Swelling of the abdomen (overfeeding, constipation and dropsy)

Fish can become swollen for several reasons. Overfeeding is one reason, in which case skipping food for a day or two should result in a quick recovery.

Constipation can cause swelling of the abdomen as well as problems with swimming and balance. This is primarily a problem with herbivorous fish (such as severums and goldfish) that aren’t given the fresh green foods they need. Constipation is easily relieved by not using dried foods, and only offering suitable laxatives: fresh green foods (tinned peas are ideal) and wet-frozen or live brine shrimps and daphnia. Epsom salt (magnesium sulphate) is a mild muscle relaxant and speeds up the effect of laxative foods; dose at 1-3 teaspoons per 20 litres.

Dropsy (or oedema) is the most dangerous type of abdominal swelling. It can be distinguished from the other problems by the way the scales on the body stick out from the body, so when viewed from above the fish looks a bit like a pine cone. Dropsy is typically a symptom of chronic bacterial infections and is therefore most likely caused by chronically poor environmental conditions. It isn’t usually treatable, and affected fish are best euthanised.

White, stringy faeces (Hexamita)

Cichlids are particularly prone to infections of the gut that cause them to produce long, stringy white faeces that are more mucous than solid waste. The likely cause is the protozoan parasite Hexamita that lives inside the gut normally without doing any harm. It becomes problematic when the host fish is stressed, typically because of poor environmental conditions. In the case of cichlids, a nitrate level above 20 mg/l is one such factor, and a particular problem in heavily stocked, poorly maintained African cichlid community tanks.
Hexamita is difficult to cure, with the antibiotic metronidazole being the only reliable treatment. Without prompt treatment, infected fish usually die within a few weeks. In the UK at least, metronidazole is a prescription-only drug that has to be obtained from a vet.

Small pits on the head and lateral line (hole-in-the-head disease)

Hole-in-the-head disease (HITH) is likely related to Hexamita infections, as described above. Although the causes and treatment are similar, the symptoms are distinctive: small pits appear on the head, and over time these pits get bigger and eventually infected. Hole-in-the-head is related to another disease known as head-and-lateral-line-erosion (HLLE) disease.

Like Hexamita infections, both HITH and HLLE are related to poor environmental conditions. Another factor appears to be inadequate diet, particularly when herbivorous fish do not receive enough fresh green food. Without treatment, the fish eventually weakens and dies from some secondary infection. As with Hexamita, the only medication that works reliably is metronidazole, a prescription-only antibiotic.

Sick Neons

Neon tetras can suffer from all the same diseases as any other fish, but they are also afflicted by neon tetra disease. Symptoms include loss of colour, poor swimming ability, disinterest in food, and commonly segregation away from the rest of the school of neons. Affected fish will often be seen on their own lurking behind plants. The disease is believed to be caused by a protozoan called Pleistophora hyphessobryconis, though in some cases the disease may be a bacterial infection.

Whatever the pathogens involved, neon tetra disease is highly contagious and quickly kills affected fish. Healthy fish likely pick up the disease from being kept in tanks with sick fish, so as soon as one fish develops neon tetra disease, it should be removed at once and euthanised. There is no cure for neon tetra disease. Neon tetras are most likely to suffer from neon tetra disease, but sometimes other fish can catch the disease as well.

Sick Dwarf Gouramis

Dwarf Gourami Iridovirus is a viral disease that affects dwarf gouramis (Colisa lalia) in all its forms and hybrids, including things like robin gouramis and cobalt gouramis. The symptoms include lethargy, loss of colour, loss of appetite, the appearance of bloody sores on the body and eventually dropsy and death. Dwarf Gourami Iridovirus is highly contagious, always fatal, and currently untreatable. Infected fish should be immediately removed and euthanised.

Mollies that don’t swim properly or rock from side to side

The shimmies is a problem where fish, usually mollies, have trouble swimming. They seem to swim on the spot, treading water or rocking from side to side. Sometimes other problems, such as finrot, will be apparent as well. In short, mollies react to poor environmental conditions in a variety of ways, the shimmies being one of the commonest. If the only problem is the shimmies, and there are no signs of finrot or fungus, simply improving water conditions should help.

Mollies are most likely to get sick in soft, acidic water. Even in hard, basic water they are sensitive fish, and non-zero nitrite and ammonia levels cause severe stress, and nitrate levels above 20 mg/l cause them significant stress in the long term. When maintained in brackish water they are generally much hardier, and if mollies are the only fish being kept, converting the tank to a slightly brackish one (around SG 1.003) will make a huge difference.


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