The A-Z of Fish Health, Part 1: A-E


Dr Peter Burgess, Senior Consultant of the Aquarian Advisory Service, takes us through the alphabet, looking at the common – and not-so-common – health matters that can affect our fish.





Condition caused by exposure to acidic pH levels that are below the tolerance range of the species in question. A large and sudden drop in pH can result in fish developing tremors and excitable behaviour such as darting and jumping. Potentially lethal. Such rapid pH falls are more likely to occur under softwater conditions where the low mineral content of the water is inadequate to stabilise the pH.

More commonly, the pH fall is gradual, causing fish to secrete excess mucus, evident as a slimy-skin appearance. Excessive mucus production by the gills leads to breathing difficulties, gasping and rapid gill beats. Fish that are kept below optimum pH range may have poor growth and fail to spawn.

Acidosis can be prevented by routinely testing the pH and by using appropriate pH buffers. Regular partial water changes also help. Before buying fish, find out whether they are suited to the pH conditions of your aquarium.


Dye-based drug that combats certain bacterial, fungal and protozoal infections of fish. Less commonly used these days due to resistance by many common types.

of bacteria.

Acute infections

Infections that are relatively short in duration, typically lasting a few hours to several days, and/or severe.


Genus of bacteria commonly found in freshwater habitats, including aquariums and ponds.  Some Aeromonas species are potentially harmful to fish, causing skin bruising and reddish or grey skin ulcers. Prompt treatment is necessary to prevent the infection from becoming life-threatening.

Mild cases may respond to anti-bacterial remedies from the aquarium store. Stubborn or severe ulcerative infections require antibiotics from the vet. Overcrowding and unhygienic water conditions will render fish more prone to infections.  

Ammonia poisoning

The digestion of proteins by fish results in the production of ammonia. Ammonia is highly toxic to fish, hence they must eliminate this waste product from their bodies by excreting it into the water. If levels of ammonia in the water get too high, the fish will be poisoned.  

Ammonia damages the skin, gills and other tissues, causing a variety of symptoms that may include breathing difficulties (fast gill beats, gasping); erratic swimming and sudden darting; skin bruising (haemorrhaging) and over-production of mucus (opaque patches on the skin). The toxicity of ammonia increases with pH and with water temperature. Levels are usually kept in check by the biological filter, which harbours bacteria that break down ammonia into less toxic substances.

Ammonia poisoning is largely preventable by routinely checking levels using a test kit and avoiding overcrowding or over-feeding. Dangerous levels of ammonia must be dealt with quickly, for example by performing large water changes and/or using an ammonia-removing agent, such as zeolite. These quick-fix measures buy time while the underlying cause is addressed.


Condition caused by a deficiency of red blood cells or a lack of haemoglobin*. The gills of anaemic fish are pale, unlike those of healthy fish which are bright red. Severe anaemia may lead to breathing problems, manifesting as gasping or fast gill beats. Bear in mind that breathing difficulties are more likely due to damaged or infected gills.

Anaemia can have many underlying causes. For example, certain bacterial infections disrupt the fish’s red blood cells, causing haemolytic anaemia.

Anaemia is also caused by a significant loss of blood, known as haemorrhagic anaemia, which can arise from an injury or from viral infections such as SVC (Spring Viraemia of Carp). Heavy infestations of blood-sucking parasites such as fish lice and leeches are another cause. Feeding a poor-quality diet that lacks certain nutrients, such as pantothenic acid, may also lead to anaemia.

*Haemoglobin is a red pigment molecule within red blood cells. Its function is to transport oxygen around the blood.


Living without oxygen. Some types of bacteria are anaerobic, for example those that cause stagnant areas of gravel within an aquarium to turn black.


Chemicals that calm fish or render them unconscious, eg for surgery. Suitable fish anaesthetics include TMS (tricaine methane sulphonate) and benzocaine. Most anaesthetics are thought to possess analgesic (pain-suppressing) properties.


Drugs that combat helminth (worm) parasites such as flukes and tapeworms. Some are available only on prescription, from a vet.


A group of chemicals that can kill or inhibit bacteria and certain other micro-organisms. Natural antibiotics are produced by moulds, eg the Penicillium mould produces the antibiotic penicillin.

Nowadays, synthetic antibiotics such as enrofloxacin and oxytetracycline are used to combat bacterial infections of fish.

Appetite loss

This often indicates a health problem, perhaps due to disease, stress (eg bullying by tankmates) or poor water conditions, including low oxygen problems. Investigate these possibilities and always perform water tests when fish go off their food. In some cases, appetite loss may be normal, for example pond fish tend not to feed during the cold winter months.

Newly introduced fish may be reluctant to feed for the first day or two until they grow accustomed to their surroundings. Sometimes the choice of food is to blame. For example, predatory fish such as gars and pikes are unlikely to accept non-living foods.     

Ascites (ascitic fluid)

An abnormal accumulation of fluid within the body cavity. The build-up of ascitic fluid leads to the condition known as dropsy in which the fish’s body bloats and its scales stick out.

Ascites is generally the result of damage or infection of the liver, kidneys or other organs that play a role in osmoregulation*. The underlying cause is often a bacterial infection, but sometimes a viral or parasitic infection, or simply old age, is to blame.  

This condition is often fatal and is very difficult to cure. Where a bacterial infection is suspected, antibiotics, from the vet, may sometimes be effective.

*Osmoregulation: the physiological process by which a fish maintains its internal salt-water balance.


Primitive single-celled organisms. They are microscopic, ranging in size from about 0.5 to 10 microns (one micron is one-thousandth of a millimetre).

Thousands of species inhabit aquatic environments, however only a small percentage are harmful to fish.  Many perform useful roles in the aquarium and pond by consuming organic waste matter, thereby helping to keep the water clean. Important among these are the filter bacteria that break down the fish’s toxic ammonia wastes into less toxic nitrite and finally to weak toxic nitrate.

Some aquatic bacteria have a Jekyll and Hyde lifestyle in that they live harmlessly within the water or inside the fish’s gut, but sometimes cause disease if the fish is weakened, eg as result of injury or stress.     

Bacteria reproduce by binary fission, in which the bacterial cell divides into two. Under optimal conditions, some bacteria are capable of dividing every 60 minutes or so, such that a single bacterium can yield millions of progeny within just 24 hours! This explains why some bacterial infections, if not promptly treated, may quickly overwhelm a fish.

Bacteria are relatively featureless in appearance, even when viewed under a high power microscope (x1000 magnification). Hence it is not possible to precisely identify bacteria simply by looking at them.

They do, however, vary in shape: many species that infect fish are rod-shaped. Others are spherical, known as 'cocci'; comma-shaped, eg. Vibrio bacteria; or spiral. Some bacteria are able to swim by waving their hair-like flagellae to propel themselves through the water.    

Bacterial diseases

Fish-pathogenic bacteria include various species of Aeromonas, Vibrio, Flavobacterium and Mycobacterium, to name just a few. Between them, these bacteria cause various diseases such as ulcers, fin-rot and tuberculosis.

Many common bacterial infections can be treated using liquid remedies obtainable from the aquarium store.

Serious or persistent infections may require antibiotics from a vet.

Overcrowding and unhygienic water conditions will render fish more prone to bacterial infections.   


Bacterial infection of the blood. Not easy to diagnose as the outward symptoms are vague and vary according to the severity of infection and the species of bacteria involved. Affected fish are likely to be very ill and listless.

Antibiotics offer the best chance of treating this life-threatening condition.  


A drug that destroys bacteria.

Balance (loss of)

Affected fish lose their ability to maintain normal swimming posture. Some may lie on their sides, turn upside-down, or float at the water surface.

Loss of balance can have many causes. A severe illness or poisoning may sometimes be to blame. In the case of fancy goldfish (such as Orandas and Moors), their stumpy body shape makes these fish prone to swimbladder disorders that lead to balance problems. The gas-filled swimbladder, which functions as a buoyancy device, may collapse, causing the fish to flounder on the substrate.

In other cases, the swimbladder may abnormally over-inflate, forcing the fish to float helplessly at the water surface. Unfortunately, swimbladder problems are hard to treat.

Air gulping is another cause of imbalance in fish: goldfish in particular may feed greedily at the water surface and take in lots of air. The entrapped air causes the fish to rise to the surface where it may stay for several hours until the gas is eventually evacuated. Air gulping can largely be avoided by briefly (couple of seconds) holding the dry food (eg flakes) in the water before releasing them, so the fish have to feed in mid-water.    


Chemical used for sedating or anesthetising fish.

Black spot disease

This disease manifests as tiny black spots beneath the fish’s skin. It is caused by the larvae of digenean flukes, eg Posthodiplostomum sp. Each black spot is an individual fluke, typically 1-2 mm diameter, that has lodged beneath the fish’s skin or muscles. Its black colour is due to infiltration by dark pigment cells that are produced by the fish in response to the infection.  Heavily infected fish may be covered with hundreds of black spots. 

Fortunately, this disease is more unsightly than harmful.  

The flukes have complex lifecycles: they must infect, in chronology, a bird, then an aquatic snail, and then a fish to complete each generation.

The necessity for a bird host means that black spot disease is non-contagious under aquarium conditions, so any affected fish do not require isolation and generally live an otherwise normal life.

Sometimes ponds that are frequented by fish-eating birds may acquire these flukes via birds’ droppings. In such cases, the parasite’s lifecycle can be interrupted by eradicating the intermediate aquatic snail hosts.


Loss of sight can arise from eye damage or eye loss, or from damage to the optic nerves or brain. In the case of damaged eyes, the underlying cause may be physical (eg damage as when a startled fish rubs against a rock) or disease. Eyes may be lost as a result of attack by aggressive tankmates. The blind fish may still be able to navigate and locate food thanks to other sensory organs such as the lateral line system that allows it to detect its surroundings.  

Many species of cave fish, such as Astyanax cave tetras, are naturally blind – at least when adult. Blindness will, however, severely debilitate predatory fish such as pikes and gars that rely on sight to locate and catch prey.

Fish with recently lost or damaged eyes should be temporarily isolated and their water medicated with a bacteria remedy to reduce the risk of infection until the injury heals.


Abnormal swelling of the body, typically caused by an accumulation of fluid within the fish’s tissues. It is generally the result of damage to the kidneys or other organs involved in osmoregulation (the process by which fish maintain correct levels of salts and fluids within their bodies).

Often, a bacterial or viral infection is responsible for the organ damage, but in some cases old age, which causes organ failure, may be to blame.  

With goldfish, a bloating condition known as 'Kidney enlargement disease' manifests as swelling of the flanks.

This life-threatening disease is caused by microscopic Hoferellus carasii parasites that invade the goldfish’s kidneys, causing these organs to enlarge considerably. Fish acquire Hoferellus by eating aquatic worms, eg Tubifex, that may harbour these parasites.

The closely-related Hoferellus cyprini causes similar problems in Koi. Isolate suspected cases to prevent these fish from shedding the parasite’s spores into the water.

Most forms of bloating are hard to cure, although a course of antibiotics may sometimes be effective where bacteria are to blame. There are no established treatments for Hoferellus infections, although toltrazuril and fumagillin hold promise.  

Blood parasites

A few species of parasite live within the blood of their fish hosts. Occasionally found in wild-caught fish are blood-parasitic protozoa such as Trypanoplasma and Trypanosoma sp. These parasites are transmitted by fish-parasitic leeches, so are unlikely to be present in aquarium-raised fish. Mild infections may cause no outward symptoms.

Heavily infected fish are listless and become thin, though these vague symptoms are more likely the result of more common diseases such as bacterial infections. Confirmation of blood protozoa requires the microscopic examination of a drop of blood.

There is no treatment.

Another blood parasite is the blood-fluke Sanguinicola. The adult flukes infect the major blood vessels and sometimes the heart chambers of carp-like fish, including Koi.

The fluke’s triangular eggs are released into the blood where they lodge in the fine capillaries of the gills and other organs.

The larval flukes hatch and migrate through the gill wall to reach the water. They then develop within an aquatic snail, emerging as infective 'cecariae' that must locate and penetrate a suitable fish host.

The damaging effects of Sanguinicola on the gills can be life-threatening, particularly in juveniles. Adults obstruct the blood vessels.

Treatment is problematic, however worming agents such as Praziquantel have been used with some success. The parasite’s lifecycle can be broken by eliminating aquatic snails from the system.


A rarely encountered fungus that causes gill rot in Koi and other members of the carp family.

Infected gills develop grey-brown mottling and localised erosion and fish suffer from breathing difficulties.

Outbreaks are more likely at water temperatures above 20ºC/68ºF, especially within unhygienic, overcrowded ponds. No effective treatment.

Brown blood disease

A serious condition caused by high levels of nitrite in the water. Nitrite binds to haemoglobin molecules within the blood cells, causing the blood to turn brown.

Nitrite inhibits the oxygen-carrying function of haemoglobin, hence badly affected fish may die from a lack of oxygen to their tissues.

A quick-fix measure is to perform one or more largish water changes and/or add a small amount of sodium chloride to the water (0.1 g/l) to reduce nitrite toxicity. Ensure there is adequate aeration.

These actions buy time while the underlying cause, usually a filter problem, is rectified.  


Certain catfish, notably plecs and their relatives, may sometimes rest on the heater, particularly if it is positioned horizontally. This can result in skin burns should the heater come on while the fish is present, so fit a heater guard.

Burn victims should be isolated in hygienic water containing a bacteria-remedy to help prevent infection of the damaged skin.  

Camallanus worms

Small (1-2 cm/½”-¾”) red-brown parasites belonging to the roundworm group. They live within the intestines of a wide variety of fishes. Livebearing species such as Guppies, platies and mollies seem particularly susceptible. It is only when one or more female worms protrude out of the fish’s anus in order to shed their eggs with the fish’s faeces that a Camallanus problem is first noticed.  

Fish may acquire these worms through eating the faeces of a Camallanus-infested fish. Another route of transmission is through eating a copepod (a relative of the Daphnia water flea) that is carrying the larval stages of Camallanus.

Mild infestations may cause no obvious ill-health problems. Fish that harbour many worms, however, may become emaciated and even die. General parasite cures will not eradicate them. Instead, a special anthelmintic drug is required, such as Fenbendazole or Levamisole. Fenbendazole can be added directly to the tank as a long-term bath, redosing every seven days for a total of three weeks. The whole tank must be treated in order to destroy Camallanus in the water.  

Note: The anthelmintic Ivermectin, which is used to treat cattle parasites, is not suitable, being toxic to fish.

Capillaria worms

These parasites are types of roundworm. They have very thin hair-like bodies reaching 5 mm - 2 cm/¼”-¾” in length, depending on the species.

Capillaria reside within the intestines of various fish, including Discus and Angels, particularly wild-caught specimens. The adult worms shed their eggs with their host’s faeces.

Mild infestations cause no obvious symptoms, however heavy worm burdens can result in appetite loss and emaciation, leading to death. Capillaria-infested fish may darken in colour – as observed in Discus. Affected fish produce only small quantities of faeces that appear abnormally thin and translucent.

Under aquarium conditions, fish acquire these nematodes by accidentally eating the larval stages that are shed into the water by infested tankmates. Tubifex worms are also capable of carrying Capillaria. Treatment requires a special anthelmintic drug such as Fenbendazole, Levamisole or Piperazine. Depending on the chosen drug, the anthelmintic is either mixed with the fish’s food or added to the water. Isolate infected fish to avoid them passing on their nematodes to others.


Whitening of the lens of the eye, causing impaired vision. One or both eyes may be affected.

In large fish, it may be possible to see the abnormally opaque lens beneath the transparent pupil of the eye – a strong hand lens helps.

Do not confuse this with clouding to the outer surface of the eye, which may indicate a bacterial or fungal infection.

Cataracts have various causes, including old age and exposure to water temperatures that are well below the fish’s optimum range.

Poor diets that lack certain essential nutrients (eg riboflavin, manganese or zinc) can also lead to this eye condition.

Additionally, cataracts can stem from long-term exposure to strong UV irradiation, as may occur when outdoor fish are kept in shallow ponds that lack shade.

Infection with the eye-fluke, Diplostomum, can also lead to cataracts.  

Cataracts in fish are incurable.

Cauliflower disease

Common name for the relatively harmless skin disease known as Lymphocystis. It’s caused by a virus.


Microscopic heart-shaped parasites that infect the skin and gills of fish.  More common in pond fish at very low water temperatures, below 10ºC/50ºF. Treat with a skin parasite remedy, added to the whole pond.

Chronic infection

An infection that persists for a long time – weeks, months or years.

Ciliate parasites

Single-celled parasites that are typically covered in numerous hair-like structures known as cilia, which are used for movement. Most ciliates are microscopic in size.

Several important skin parasites of fish belong to this group, including: Ichthyophthirius (whitespot parasite), Trichodina, Chilodonella, Tetrahymena, Brooklynella and Uronema.     

Clamped fins

Condition in which the fins, notably the dorsal and anal, are held tightly to the body, and the tail fin may not be spread out normally. If a fish clamps its fins for most or all of the time, this generally indicates ill health. Fish that are stressed or victims of attack often exhibit clamped fins. Brief bouts of fin clamping, however, may be normal, and is used by some fish as a means of communication.

If many fish within the aquarium or pond show this condition, then investigate for a water problem or widespread disease (such as a bacterial infection or outbreak of skin parasites). 

Cloudy eye

Whitish film or growth covering part or all of the eye surface. It can affect one or both eyes. Often, only a single fish is affected.

This condition has many causes. For example, a bacterial infection of the eye might be to blame. In some cases, it is due to a fungus infection that typically manifests as white-grey fluffy growths that project outwards from the eye.

In many cases, an underlying water problem, eg poor hygiene, is a contributory factor. Feeding a poor quality diet that lacks certain vitamins, eg vitamin A or C, can also lead to cloudy eye.

Treatment involves making any necessary improvements to the water conditions or diet, or medication of the water with a bacteria or fungus remedy.

If just one or a few fish are affected, they can be treated in isolation.         

Columnaris disease

A potentially serious bacterial disease. Caused by Flavobacterium columnare, formerly known as Flexibacter columnaris. Flavobacterium infections manifest in many ways, including the condition known as mouth fungus.

In some cases, this bacterium targets the gills, skin or muscles. Flavobacterium columnare also cause tail rot in which the tail fin erodes to the base, and may rapidly progress to the fish’s body, causing a white-pinkish discolouration of the infected tissue.

Adverse conditions such as poor water quality, stress or overcrowding can make fish prone to columnaris disease. Flavobacterium infections may prove fatal if not treated at an early stage with a commercial bacteria remedy. Advanced or stubborn cases require antibiotics from the vet.

Copper sulphate

A blue crystalline chemical that is sometimes used to eradicate skin parasites in marine and freshwater fishes. It is highly toxic to certain invertebrates.   


Ingestion of faeces. Many fish engage in this unpleasant feeding behaviour. Fish can become infected with certain parasites, eg Camallanus worms, and bacteria, eg mycobacteria, as a result of feeding on the faeces of other infected fish.


Hormone found in the blood of fish and other animals. The levels of cortisol increase with stress. High cortisol levels reduce the fish’s immune defences, hence badly stressed fish are more prone to infections.


Genus of parasites known commonly as gill flukes. Dactylogyrus live on the surface of the fish’s gills, but occasionally migrate to the surrounding skin. They rarely exceed 2 mm in length hence are invisible to the naked eye. These parasites possess sharp hooks that abrade and damage the gills, causing their host to suffer breathing difficulties.

Badly affected fish may exhibit fast gill beats, gasping at the water surface, and sometimes flared gill covers. Heavy infestations are life-threatening. The fluke-damaged gills are prone to secondary bacterial and fungal infections.  

Dactylogyrus outbreaks can be treated using a proprietary fluke remedy that is added to water. They lay eggs that are resilient to chemical attack, hence repeat treatments may be necessary to destroy the young flukes as they hatch.


Common deformities in fish include bent backbones, stumpy bodies, misshapen heads, curled gill covers or missing fins. Deformities can have many causes and may affect the fish at any stage in its life.

At birth/hatching: Within a brood it is normal for a few individuals to be deformed in some way. Their chances of survival depend on the nature of the deformity. In the wild, survival is less likely, due to the increased risk of predation and other selective pressures. Where a significant proportion of the brood is deformed, say over 5%, this could indicate a genetic defect in one or both parent fish, or adverse environmental conditions under which the eggs or fry were raised.  

Genetic defects are often the result of in-breeding. Adverse environmental conditions that can cause abnormal development of eggs or fry include low oxygen levels, extremes of water temperature, overcrowding and high levels of heavy metals, eg cadmium.

Studies on cultured food-fishes have revealed that nutritional deficiencies in the broodstock (especially the female fish) can cause fry abnormalities.

Deformities in later life: These can be caused by disease, injury or tumours. Certain chronic diseases, such as mycobacterial infections, weaken the fish’s backbone, leading to abnormal body curvature.

In Koi, a disease caused by tiny parasites (Myxobolus sp) causes a pronounced swelling around the face and gills, giving it its common name: ‘frog-face’ disease. Physical injuries may lead to backbone fractures, resulting in the fish – if it survives the trauma – having a bent back. Large tumours of the skin or internal tissues can result in a misshapen or lop-sided body.     

Prevention and treatment: Unfortunately, most deformities in fish are impossible to rectify. Preventative measures rely on good fish husbandry: optimal water conditions, a disease-free environment and a healthy diet.

In-breeding defects can be largely avoided by occasionally introducing unrelated fish to the breeding stock  to refresh the gene pool.


Genus of fungus-like parasites. One species, Dermocystidium koi, affects goldfish and Koi, causing smooth raised lumps to develop on the skin or fins. The lumps eventually burst, resulting in open sores that will heal.

Dermocystidium is contagious but fortunately not common. No known chemical cure.

Digenean flukes

Large group of parasitic flatworms. Some are parasites of fish, such as the eye fluke, eg Diplostomum, and blood fluke, Sanguinicola.


Potentially fatal condition in which the fish’s body swells up. It is caused by an abnormal accumulation of fluid within the body and is often accompanied by raised scales and sometimes pop-eye. In addition to their bloated appearance, affected fish may become sluggish and go off their food.

Dropsy is generally the result of damage or infection of the kidneys or other organs that play a role in osmoregulation, or the internal salt-water balance. Internal infections with certain types of bacteria and viruses, eg Spring Viraemia of Carp virus, can lead to dropsy. Some internal tumours, or simply organ failure through old age, can also give rise to this condition.

Dropsy can be difficult to treat partly because it has many underlying causes, some of which, eg internal tumours, viral infections and organ failure, are incurable. Also, by the time the fish develops signs of bloating, severe internal damage may have already occurred. Hence, over-the-counter remedies for dropsy cannot be guaranteed to work in all cases. Antibiotics from the vet offer the best hope in situations where a bacterial infection is suspected.


Technical term for "egg bound" in which a female fish is unable to release her mature eggs. This condition can arise if there are no adult males for the female to spawn with, or if the water conditions are unfavourable, eg a cold spring or summer may inhibit spawning in pond fish. Generally, dystocia is not harmful and the retained eggs may eventually be reabsorbed.


A parasite that lives on the surface of its host, eg on the skin, fin or gill surface. Examples are the skin flukes such as Gyrodactylus and various skin-parasitic protozoa including Trichodina, Chilodonella and Ichthyobodo.  

Egg fungus

This fungal condition manifests as grey-white fluffy growths that project outwards from the egg surface. It is caused by various aquatic moulds, such as Saprolegnia, which can also cause fungus problems on fish. Infertile eggs (which typically turn white) are particularly prone to fungus attack. However, fertile eggs can also be affected, risking death to the developing embryos within.

Fish that practice parental care, eg cichlids, routinely clean their eggs to reduce the risk of fungus attack.

Over-the-counter fungus medications, such as methylene blue, can be added to the egg-incubation aquarium as a means of prevention. Warning: methylene blue destroys the filter bacteria!


Certain diseases or poor diet can cause fish to lose body weight and become progressively thin. Emaciation often presents as a hollow belly in which the fish’s abdominal region looks pinched in, sometimes with a concave profile.

Common causes of emaciation include: dietary deficiencies, feeding incorrect foods, chronic loss of appetite, chronic infections such as caused by mycobacteria (fish TB), tapeworms or nematode worm infestations of the gut. 


A parasite that lives inside its host, eg in the gut; within the internal organs; muscles or blood. Examples are the gut-dwelling tapeworms and the skin or muscle-dwelling digenean flukes. The whitespot organism (Ichthyophthirius) is also an endoparasite because it lives just beneath the skin surface.  


Synthetic antibiotic belonging to the quinolone group. Marketed as Batril®. Considered a useful treatment for certain bacterial infections in ornamental fish. It can be given via the water or incorporated with the fish’s food.  


Genus of single-celled protozoans. Normally free-living, these tiny, sessile (immobile) organisms sometimes attach to the body of fish, particularly ‘hard’ surfaces such as the fin rays.

For example, Epistylis may colonise the tips of the pectoral fins of catfishes, including Corydoras.

Although not true parasites, Epistylis may elicit an inflammation response by the fish which sometimes leads to localised ulceration that is vulnerable to bacterial infection.

A large colony of Epistylis appears to the naked eye as a small tuft of growth on the fish, such that it can be mistaken for a fungus infection.

Under the microscope, however, Epistylis reveals its characteristic cylindrical or bell shape and its long stalk for attachment: this contrasts with the branching root-like structure of fungus.

Unhygienic water conditions favour the proliferation of Epistylis. Treat affected fish with a general skin parasite cure, for instance, a whitespot cure.


Genus of parasites, comprising over 100 species. They are primitive crustaceans (distant relatives of crabs and shrimps) that invade the body surfaces of various freshwater and marine fish. They range in length from 1.5 to 6 mm. One important species is Ergasilus sieboldi (the gill maggot) which infests the gills of cichlids, the carp family and certain other fish.

Ergasilus sieboldi is rarely a problem under aquarium conditions, being more commonly encountered in pond fish. Infestations of the gills can be life-threatening, causing the fish to suffer breathing difficulties, manifesting as fast gill beats, gasping and surface gulping. 

Specific anti-crustacean treatments, eg from the vet, are required to eradicate them.  


Antibiotic administered with the food or via the water. Warning: erythromycin can destroy the biological filter bacteria, and prolonged treatment may cause kidney damage in fish, as shown with salmonids.


Abnormal protrusion of one or both eyes. This condition, which can have many causes, is known commonly as 'pop-eye'. This will be covered in more detail under the entry 'Pop-eye'.

Eye fluke (Diplostomum)

As its common name suggests, this tiny worm-like parasite invades the fish’s eyes, sometimes causing impaired vision, even total blindness.

Diplostomum has a complex life-cycle in which the parasite must sequentially infect a bird, an aquatic snail and then a fish in order to complete one generation. The involvement of birds (notably gulls, which harbour the adult flukes in their guts) mean that Diplostomum is generally only a problem in pond-reared or wild-caught fish.

The larval flukes that are shed by infected water snails must locate a fish and penetrate its skin. Once inside the fish, they migrate via the blood system to the eye.

Using a strong magnifying glass, the young flukes of 0.5 mm long can sometimes be seen as opaque blobs within the fish’s eye. Affected fish may have one or more flukes in one or both eyes.

Diplostomum infections are difficult to treat, but fortunately are not very common and only rarely life-threatening.

One method of prevention is to eliminate all aquatic snails from the pond – easier said than done – in order to break the fluke’s life-cycle.

Eye problems

The eyes of most fish are similar in structure to human eyes, and they share our ability to see in colour. The lack of eyelids in bony fish, as compared to sharks, means that the eyes have no protection against the environment.

Eye problems in fish include bulging eyes (exophthalmia), cloudy eyes and various physical injuries.

  • Infectious diseases of the eyes: Fortunately, very few disease-causing organisms specifically target the eyes of fish. One notable exception is the eye fluke that spends part of its lifecycle as the larval stage within the eye lens and eye fluid.Certain surface-dwelling parasites, such as the whitespot parasite, Ichthyophthirius, and skin flukes, Gyrodactylus, sometimes settle on or crawl over the eyes, causing inflammation. Under certain circumstances, water moulds (eg Saprolegnia fungus) establish on the eye surface, causing cotton wool-like growths. Various types of bacteria, including Aeromonas species, may cause eye problems including pop-eye.  If left untreated, eye infections can lead to reduced vision or total blindness. Fortunately, visually impaired fish retain some ability to navigate and locate food, thanks to their other sensory systems: these fish may, however, require isolation otherwise they could lose out at feeding times if kept with active tankmates. Mild eye infections can usually be treated with a suitable remedy, eg fungus or bacteria remedy, added to the water. Stubborn bacterial infections, including some cases of pop-eye, may require antibiotics from the vet.
  • Non-infectious diseases of the eyes: Many eye problems in fish stem from unhygienic water conditions or poor diet. Exposure to very low water temperatures may damage the proteins within the eye, leading to clouding of the lens. Similarly, exposure to strong sunlight, such as when outdoor fish are held in shallow ponds that lack shade, can lead to cataracts. Dietary-related eye problems may arise if the fish is given poor-quality foods that lack vital nutrients important for eye development and function. For example, a dietary deficiency of vitamin D can lead to pop-eye. Where the eyes are damaged, for instance via cuts or grazes such as caused by handling or net injuries, the addition of a broad-acting bacteria remedy will help prevent infection while the eye heals.
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