The A-Z of Fish Health, Part 3: M-R


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.





Dietary condition often caused by inadequate food intake or poor diet. Sometimes due to certain diseases, such as a heavy tapeworm infestation. Affected fish slowly lose body mass and muscle wastage may also occur.  

Malnourished fish become emaciated and typically develop a pinched-in 'hollow' belly. As it becomes progressively thinner, the fish’s head may appear disproportionately large compared to the body.  

Fish may become malnourished because they refuse to feed, perhaps due to ill-health or stress, or because they are inhibited by aggressive tank mates.

Offering the wrong types of food can also lead to malnutrition. For example, some fish — such as pike, seahorses, and the fry of many species — may only accept living foods and refuse to eat at all if only offered dead or artificial diets.


Veterinary drug used to treat skin and gill flukes (monogenean flukes). Can be delivered via the water at levels of 1 mg/l per 24-hour bath. Higher doses may be required to combat some species of monogenean.  Available on veterinary prescription.  


Tumour of the black pigment cells within the skin and affected fish develop slightly raised black areas on the surface.  

Some platy/ Swordtail hybrids are particularly prone to melanomas which are also occasionally seen in ageing tetras. Usually lethal, but non-contagious. There’s no known cure.

Methylene blue

Dye-based chemical traditionally used to treat fungal, bacterial and protozoal infections. Less commonly used these days, except for treating egg fungus). May destroy the biological filter bacteria.


Veterinary drug, marketed as Flagyl®.  The drug of choice for treating hole-in-head (HIH) syndrome in cichlid fishes. Metronidazole combats intestinal flagellate protozoa such as spironucleus and hexamita which are implicated in HIH.  

It’s also effective against certain anaerobic (oxygen-hating) bacteria.

Can be administered via the water or with the fish’s food, assuming they are feeding normally. Seek expert advice regarding dose rates and exposure times.  

Not readily soluble in water so ensure the drug is fully dissolved before use.  

Some experts switch to using di-metronidazole when dealing with stubborn cases of HIH.   


Large group of protozoan parasites that affect various animal groups, including freshwater and marine fish.

Microsporidians are very small, spore-producing organisms that live within the cells of their host as intracellular parasites. The tiny spores (less than one-hundredth of a millimetre) may remain viable in the water for long periods, sometimes over a year.  

Numerous species affect fish, perhaps the best known being Pleistophora hyphessobryconis  which causes neon tetra disease. Another species, Glugea anomala, causes the small lumps or cysts we often see on the skin of wild sticklebacks. There ‘s no established treatment for microsporidial infections.


Group of parasitic flukes. Most species live on the body surfaces of cold-blooded animals, particularly fish and amphibians. They include the skin flukes, such as Gyrodactylus, and gill flukes (Dactylogyrus), both of which cause serious damage to pond and aquarium fish.

Some monogeneans are highly host-specific, affecting only one or a few fish species.

Mouth fungus

Disease causing white-greyish lesions on the mouth and they may develop into cotton-wool like growths.

Despite its common name and fungus-like appearance, this disease is caused by a bacterium (Flavobacterium columnare).  

The lesions can rapidly advance and prove fatal if not treated promptly. Use a specific mouth fungus remedy or other suitable anti-bacteria treatment.  


Genus of rod-shaped bacteria.  Some are harmless free-living organisms, others cause disease in humans and animals. Notable species affect fishing are Mycobacterium marinum and M. fortuitum which cause wasting disease (Fish TB).

The bacteria can occur throughout the body, including skin and internal organs such as liver, spleen and kidney. Symptoms are non-specific and vary, but may include gradual weight loss, haemorrhaging and ulceration of the skin, poor coordination and pop-eye (exophthalmos). 

Advanced cases may lead to curvature of the spine.  

Treatment is difficult, as even multiple antibiotic therapy may fail. Permanently isolate affected populations and consider putting down badly diseased individuals.  Do not breed from affected fish, as mycobacteria can be passed from parents to progeny.  

Wear rubber gloves when handling suspected cases or their aquarium water as the mycobacteria can, on rare occasions, infect human skin, notably the lower hand.

Nematode worm infestations

Known commonly as roundworms, nematodes are characterised by elongate, cylindrical bodies which are non-segmented. They range, depending on species and stage, between a few millimetres to a few centimetres — just over an inch.  

Many nematode species are harmless, living freely in soil or water, and some are important live foods for fish, notably the 'microworms' cultured by fishbreeders.

Others are parasites, including some 650 species known to affect various freshwater and marine fish. Most live as adults in their host’s digestive system. Some nematodes lay eggs, others give birth to live young.  Their life cycle may be direct (from fish to fish) or involve two or more different hosts (fish and aquatic invertebrate such as Cyclops).  

Parasitic nematodes key to ornamental fish include Camallanus and Capillaria. Low level infestations, low numbers of worms, may cause no obvious signs. Heavy infestations will consume a significant proportion of the fish’s ingested food and may cause gut damage. Badly affected fish become emaciated, lethargic and may die.  

These symptoms can have other causes, such as poor diet, bacterial gut infection and other gut parasites, so confirmation of an infestation may require microscopy to check for nematode eggs within faeces. A vet or fish health expert can do this. With some species, such as Camallanus, the worms may protrude from the fish’s vent.

Treatment requires de-wormer drugs known as anthelmintics, such as fenbendazole or levamisole which can be added to the water. For fenbendazole, dose 2mg per litre for seven days and repeat twice more to complete a 21-day treatment.   

Some anthelmintics are available only on vet prescription. General parasite cures from the aquarium shop are ineffective.

Neon tetra disease

This affects Neon, Glowlight and Penguin tetras, and possibly other fish such as barbs and Zebra danios. Cardinal tetras appear unaffected.

Caused by a microscopic spore-forming parasite, Pleistophora hyphessobryconis invades the fish’s muscles, but low level infections may cause no outward symptoms.  

Disease is characterised in Neons by fading of colours, development of white patches on the skin, and damage to muscles (including localised muscle swelling) leading to bodily distortion and abnormal swimming. Badly affected fish become emaciated and die.

Fish may acquire the parasite by foraging on the carcass or ingesting the faeces of infected fish. There’s no widely accepted treatment, although toltrazuril from the vet holds some promise.

Permanently isolate suspected cases to reduce risks of spreading disease to other vulnerable tetras. If an outbreak occurs, avoid restocking with susceptible species (notably Neons, Glowlights and Penguins) for at least one year as the spore stage may remain viable in the aquarium for many months.  

Note that colour fading and development of white skin patches in Neons and other fish can also be caused by a bacterial infection (flavobacterium bacteria), which is often accompanied by tail rot.

This is known as 'false neon disease' and can be treated with a general anti-bacteria drug.   


Antibiotic, sometimes used for treating bacterial infections in fish and available as neomycin sulphate. Can be given as a prolonged bath, using 65 mg per litre. One or two repeat treatments, three days apart, may be required. This drug destroys filter bacteria, so monitor ammonia and nitrite levels during and after treatment.


Commonly known as a 'tumour', a neoplasia arises when one or more cells within the fish’s body begin to malfunction and multiply out of control. The abnormal cell mass may slowly grow to a visible lump.  

Neoplasias can arise in virtually any organ or tissue and some are more harmful than others.

Those associated with the skin include the generally harmless papilloma forms sometimes seen on goldfish and other species. Potentially more serious are the pigment cell neoplasias, notably melanomas.  

Neoplasias of the internal tissues may go undetected, although some grow to cause visible bulging of the body.  

Certain neoplasias are triggered by viruses and can affect several fish. Very old or chronically stressed ones are more prone.

Neoplasias are generally unpreventable and there are no chemical treatments.

New tank syndrome

This term refers to a common, but largely avoidable, cause of mass mortalities in newly set-up aquariums and can also affect ponds. Many or all fish die as a result of self-poisoning, due to a build-up in the water of their toxic wastes; ammonia and its break-down product nitrite.  

Poisoned fish may initially become jumpy and exhibit fast gill beats and/or surface-gulping. In acute cases, they may spin or appear otherwise disorientated due to neurological damage by ammonia.

The risk is greatest in aquariums stocked too rapidly with lots of fish; the tank and filter initially lacking adequate numbers of nitrifying bacteria that break down ammonia and nitrite wastes.  

The critical period is typically within three weeks of setting up the aquarium, but this may be prolonged under coldwater conditions and its duration is influenced by other factors such as stocking density.

Stock slowly — for example, adding just a couple of small fish per fortnight in the case of a tropical aquarium.

Frequently test for ammonia and nitrite levels, perhaps every few days, during the critical few weeks after setting up. If ammonia and/or nitrite levels rise, they should be promptly reduced by partial water changes and/or ammonia-neutralising chemicals available from your aquarium shop.

Eventually, both ammonia and nitrite should stabilise to virtually zero as the filter and aquarium become adequately colonised with nitrifying bacteria — marking the end of the NTS-critical phase.  

Nifurpirinol (furanace; auranace)

Synthetic antibiotic effective against a range of bacterial pathogens of fish. Can be added to the water, as in a short-term bath involving 1-2 mg per litre, up to six hours. This drug is rendered inactive by bright light and is toxic to scale-less fish, including loaches and catfishes.  It is also a carcinogen, so handle with care. Available on veterinary prescription.

Nitrate shock

Ammonia wastes are broken down by nitrifying bacteria (for example, filter bacteria) into nitrite and then nitrate. This end product is mildly toxic and some species are more nitrate-sensitive than others.  Nitrate levels slowly accumulate in the aquarium water and are generally kept in check through regular partial water changes or with special nitrate-removing chemicals, or filters.  

Poorly maintained aquariums may have very high nitrate levels (over 100 mg per litre). Fish may be able to adapt to a gradual increase in nitrate, but are less able to withstand sudden, dramatic exposure to high levels.  

Nitrate shock can occur when newly purchased fish — typically kept in low-nitrate conditions in the shop — are placed in a home aquarium containing a high nitrate level. On the second or third day after purchase they are often 'mysteriously' found dead, due to nitrate poisoning. Resident fish, on the other hand, have physiologically adapted to cope with high nitrate levels and remain 'normal'.    

Nitrite poisoning

Ammonia wastes produced by fish are converted into nitrite by certain types of bacteria, notably those inhabiting the biological filter.  Other filter bacteria convert nitrite into nitrate, but if these are absent or in insufficient numbers, then nitrite may accumulate in the water, risking harm or death.

Some species, such as Discus, are more nitrite-sensitive than others, like Guppies.  In general, the nitrite level should be kept below about 0.5 mg per litre, but ideally should be zero— being undetectable using aquarium test kits.  

Nitrite harms by affecting oxygen transportation by the blood.  As a result, nitrite-poisoned fish suffer from oxygen starvation and exhibit fast gill beats and gasping. Their gills may turn brownish.

Check for nitrite using an aquarium test kit. A high level generally indicates that the biological filter is not working properly, or not coping with the quantity of fish. As a short-term solution, the nitrite level can be reduced by performing one or more largish water changes. In freshwater aquariums, adding sodium chloride to the water (0.1 grams per litre is sufficient and is tolerated by most fish) will reduce nitrite toxicity. These measures buy time while the underlying problem is rectified.  


Genus of rod-shaped bacteria. Some cause chronic diseases in freshwater and marine fish. Important to ornamental fish, especially tropicals, is Nocardia asteroides.  

Symptoms in fish are similar to those caused by mycobacteria and may include skin ulceration and discoloration, and pale nodules (granulomas) within internal organs (spleen, kidney). Infections due to nocardia are probably less common than those caused by mycobacteria, but both types of diseases are difficult to treat, even when using antibiotics. Isolate suspected cases.

Nutritional diseases

Generally caused by prolonged feeding of a nutritionally inappropriate diet.  This may lack one or more essential nutrients, such as an essential amino acid or vitamin, resulting in the fish being unable to grow or function normally.  The diet may contain excessive amounts of a particular component (for example, vitamin, lipid) toxic to fish.

Nutritional diseases are typically chronic, perhaps taking many weeks to cause outward signs in fish. Symptoms are vague and vary greatly according to the nature of the problem, and include eye cloudiness and cataracts, skin lesions (bruising, haemorrhaging), fin erosion, spinal curvature, emaciation.

These diseases may be mistakenly attributed to other causes, such as bacterial infection, when the diet is at fault. Nowadays, the wide availability of nutritionally balanced dry diets, frozen foods, and live food cultures means that nutritional problems can largely be avoided. Commercial dry foods, such as flakes and pellets, do vary in quality between brands, so pay a little extra for a quality product.          


Gross overfeeding or the feeding of poorly balanced and/or fatty diets may lead to obesity and associated health problems. Obese fish may develop an abnormally large abdomen and increased girth.  There may be 'hidden' damage too, such as degeneration of the liver due to fatty diet.  

Obesity is commonly seen in carp (Cyprinus carpio) raised for angling purposes for which the fish have been fast grown on fatty, inferior quality foods. Covers of carp angling magazines frequently display obese specimens held out of water, their bloated abdomens sagging under the pull of gravity.

Certain catfishes seem particularly prone, again due to excessive or inappropriate foods.

However, before placing a seemingly obese fish on a diet, consider other possible causes, such as abdominal enlargement due to developing eggs or embryos, internal infection leading to fluid accumulation in the abdomen and other tissues (dropsy, typically accompanied by raised scales); kidney enlargement disease (polycystic kidney, which can affect goldfish) or internal tumour.   

Oxygen starvation (suffocation)

This life-threatening condition is caused by a lack of oxygen reaching the tissues.  Fish suffering from mild starvation may go off their food but, in more severe cases, they may exhibit fast gill beats and/or gasp or gulp at the water surface. Death can occur rapidly.  

The underlying causes of oxygen starvation can be grouped into two main categories:

Insufficient oxygen in the water: Low dissolved oxygen levels can be the result of overcrowding, pollution, or inadequate aeration.  High water temperatures will also reduce the oxygen- carrying capacity of the water. Under such conditions, often the larger individuals, and those species that require high oxygen levels, are first to suffer. For example, Orfe within the pond will show signs of oxygen starvation before goldfish or Koi.  

Gill or blood problem: Fish will be unable to extract enough oxygen from the water if their gills are badly damaged. They may be infected by bacteria, fungi, or parasites, or damaged by certain toxic chemicals in the water, such as ammonia or chlorine. High levels of nitrite in the water can lead to suffocation by diminishing the blood’s ability to carry oxygen to the tissues.

Act promptly if fish develop signs. Try and increase aeration as a short-term emergency measure. If a water problem is suspected, consider moving the fish to a spare aquarium, assuming it has suitable and safe water parameters, or perform a large water change to dilute out an offending chemical.

Perform water tests, notably for ammonia, nitrite, and Ph, and ideally for dissolved oxygen. If a gill infection is suspected, treat with an appropriate disease remedy. Seek help if in doubt.

When several or all your fish die within 24 hours, having developed breathing difficulties, then suspect a water problem rather than an infectious disease.


A type of tumour manifesting as a slow-growing flat, round, or wart-like growth on the skin or other body surface. Some papillomas can reach pea size or larger, but are mostly harmless. They are not uncommon and some species, such as goldfish, seem particularly prone.  Affected fish may have one or more of these growths, but otherwise appear normal and healthy.

There’s no chemical treatment and the growths are generally best left alone. Surgical removal by a vet may be warranted in a few cases, for example where the lump encroaches on the fish’s vent or gills. Seek help if in doubt as to whether a lump is a tumour.

Polycystic kidney

Serious kidney disease which commonly affects goldfish and Koi.  Also known as kidney enlargement disease due to abnormal development of large fluid-filled cysts within the kidney tissue.  

Affected fish develop considerable abdominal swelling which may appear asymmetrical (one side of the fish being more swollen than the other). In some cases, the enlarged kidneys displace the fish’s swim bladder, leading to buoyancy problems.  Fish ultimately die from this condition.

Outward symptoms can be confused with the bloating syndrome known as dropsy. However, there is generally no scale erection that typically accompanies this.  

Infection with the kidney parasite Hoferellus (a microscopic spore-forming parasite), has been implicated as the major underlying cause.  In some cases, a genetic defect or kidney tumour (neoplasia) may be to blame. This condition is largely untreatable and seriously debilitated fish should be put down.

However, experimental use of the anti-parasite drugs toltrazuril and fumagillin has shown some success in combating Hoferellus infections in goldfish.

Pop-eye (exophthalmos)

Abnormal protrusion of one or both eyes and more easily detected when examining the fish head on, or looking directly down from above.  This condition is generally caused by a build-up of gas or fluid behind one or both eyes, forcing them to bulge from their sockets.  

Pop-eye often accompanies the bloating syndrome known as dropsy in which the body swells and scales protrude.  Underlying causes are varied, but an internal bacterial infection is frequently blamed. It requires prompt treatment with an anti-bacterial medication or antibiotics from the vet, but generally is not highly contagious.

Certain viral infections or a dietary lack of vitamins A or E have also been linked to pop-eye and unhygienic water conditions can make fish more prone.


An anthelmintic (de-wormer) and the drug of choice for treating intestinal tapeworms and monogenetic flukes (skin and gill flukes).  Can be administered as a short-term bath, such as dosing 2-10 mg per litre, for up to six hours and provide good aeration. Perform one or two repeat baths, five days apart, if required.

For treating gut tapeworms, the drug can be incorporated with food, assuming the fish are eating. Use 5-10 milligrams of praziquantel per gram of fish food, given daily for three consecutive days.  It is available on veterinary prescription.

Respiratory stress

Condition in which fish has breathing difficulties. Those affected typically have fast gill beats and/or gasp or gulp at water surface — the latter not to be confused with surface feeding!

Common underlying causes are:

  • Insufficient oxygen in the water (environmental hypoxia), perhaps due to overcrowding, pollution or inadequate aeration.
  • Gill damage caused by water conditions (chlorine or ammonia damage, exposure to extremes of pH).
  • Gill infection caused by certain viruses, bacteria, fungi or parasites.
  • Blood problem (high nitrite levels reducing blood’s ability to carry oxygen to tissues).

Perform various water tests (notably for ammonia, nitrite, pH and, where possible, dissolved oxygen) before considering gill infection. Consider if fish have been recently exposed to tapwater not pre-treated with a chorine remover.

Tip: Where several previously healthy fish suddenly develop signs of respiratory stress, say within 24 hours, then condition is likely due to low oxygen levels or water problem rather than infectious disease.   


Group known technically as nematodes. Some are internal parasites of fish.