Fish are made up of 80% of the substance that constitutes their environment: water. Due to this, fish are always being affected by their surroundings since only a very thin, permeable membrane separates them from the water they live in. What is not commonly realized by many fishkeepers is that most of the pathogens that cause disease in fish are always present in aquarium water; they are not necessarily introduced by new fish or water. While some pathogens or parasites don't always live in aquariums, the majority of those responsible for the most common illnesses do. Examples of this are the protozoa that that feed off of tissue in a fish's body and if the fish is healthy, populations of these parasites are kept under control. It is the introduction of stressful conditions that causes the majority of disease outbreak in home aquariums.

In a mature, established aquarium with good water quality, these populations of pathogens exist in a balanced state of equilibrium with the environment that the fish live in, and disease is rarely, if ever, seen. The immune systems of the fish are able to function normally because the water they are living in supports them perfectly. However, if any water quality parameters shift, this balance is disrupted and the fish become steadily more vulnerable to infection. Factors such as temperature, dissolved oxygen levels, ammonia levels, nitrite and nitrate levels, pH, buffering capacity, and water hardness contribute to how stable or unstable a fish's environment is.

In addition to close attention to water chemistry, the physical makeup of the aquarium and overall husbandry must be considered. Are the tankmates suitable for each other? Are the fish being offered a proper diet? Are carnivorous fish being accomodated? Are vegetarian fish being fed properly? Do the fish have enough hiding spots to retreat to? Are schooling and shoaling fish being maintained in groups? Are solitary fish being overcrowded? All of these questions are important to understanding fish health and realizing why fish illness occurs.

New Tank Syndrome and Fish Disease

In new, unestablished fish tanks, disease is especially common, and first time or inexperienced fishkeepers may not know why it is happening. It is very important to have at least a basic understanding of what is referred to as New Tank Syndrome or the Nitrogen Cycle. The basic cycle is as follows:

Waste & Excess Food Ammonia/Ammonium Nitrite Nitrate

In a brand new aquarium, fish waste and extra fish food often fall to the bottom of the tank as detritus and once there, they begin to break down. In a new tank, ammonia levels remain high until Nitrosomonas bacteria establish; it can take up to six weeks for a population of these bacteria to grow in a tank so that waste breakdown is continuous. These bacteria are responsible for breaking down ammonia into nitrite. Nitrite is a slightly less toxic substance; ammonia is toxic at as little as 0.25 ppm, while nitrite levels can be higher than this before they become lethal to fish.

Ammonia damages fish in several ways. It disrupts osmoregulation, the process by which fish maintain equilibrium of body salts relative to their environment. In freshwater fish, this means that the fish will produce more urine, in marine fish, this results in the fish needing to drink more. With ammonia levels higher than 0.25 ppm, the mucus layers surrounding the gills begin to break down, causing the gills to swell. The fish then cannot take in as much oxygen as it would normally. With increased ammonia levels, the blood's hemoglobin isn't as able to carry oxygen to the body's tissues. In extreme cases, the membranes of the skin and internal organs begin to be destroyed, as well as the central nervous system of the fish. In lesser concentrations, ammonia is responsible for increasing susceptibility to gill disease, finrot, dropsy and bacterial infection.

Nitrite, while less toxic than ammonia, still has adverse effects. It breaks down red blood cells and also reduces the oxygen-carrying potential of hemoglobin. It can also damage the kidneys and liver of the fish. Nitrite levels can be helped with the addition of salt to freshwater; it will help reduce its toxicity. Due to this, nitrite poisoning is more rare in marine aquariums.

The final stage of the nitrogen cycle is the breakdown of nitrite into nitrate, the least toxic of the three. Nitrobacter and Nitrospira bacteria begin to establish in this later stage of new tank syndrome to oxidize the more toxic nitrite. High nitrate levels tend to be more toxic to marine fish than freshwater fish, as nitrate is almost nonexistant in seawater. In particular, delicate saltwater invertebrates are particularly susceptible to nitrate poisoning.

At the end of this process, the aquarium will have reached full maturity and the fish will be much less likely to develop disease, as these stressors will be largely absent from their environment in large concentrations. The nitrifying bacteria described above will colonize on nearly any surface in the aquarium, primarily in the gravel bed and in whatever type of filter is in the tank. This is why it is never recommended that fishkeepers empty out their entire tank to clean it; it would be restarting this whole process again. There are products out on the market now such as bacteria starters and products like Marineland's Bio-Spira that ensure faster cycling, but unless aided by a product like that, it is best to keep the above in mind.

New Fish & Quarantining Procedures

When introducing new fish, nearly every expert in the field of fishkeeping recommends quarantining any newly purchased fish for at least four weeks. While univerally thought of as a huge pain to do by new hobbyists, quarantining can often save your fishtank from a potentially lethal outbreak of disease. There are many parasities and pathogens than can be brought in by fish from pet stores and fish breeders, and in the long run, it is extremely beneficial to be cautious on this point. Parasities such as fice lice and anchorworm that might otherwise never be seen in your tank can suddenly become a problem of epidemic proportions.

If you are truly serious about keeping fish, it is wise to set up a quarantine tank; generally a 10-20 gallon aquarium is suitable unless you're intending on purchasing very large fish. It can be very spartan and is usually better that way so that observation of the new fish is easier. Keep gravel on the bottom to help maintain nitrifying bacteria populations and one or two hiding spots, but aside from that decor should be minimal. Use a small sponge or power filter and a heating element if necessary. For tropical fish, higher temperatures will speed up the life cycle of most parasites and pathogens and as such symptoms will show faster. Maintain temperatures in the quarantine tank at approximately 78°F during the quarantine period.

Recognizing Symptoms of Illnesses

Symptoms of disease are most often manifested early on in the fish's behavior; strange or altered behavior may be the earliest indicator a fishkeeper can have of a possible problem. Here is a list of the most common, and some not-so-common, fish diseases that occur in home aquariums.

  • Ich: "Ich" or "white spot disease" as it is alternatively known by, is caused by two different protozoa: Ichthyophthirius multifiliis in freshwater aquariums and Cryptocaryon irritans in saltwater tanks. These tiny parasites lie just beneath the skin of the fish, and will appear as round, small, white dots. Fish infected with ich may demonstrate "glancing" or "flashing" behavior where they will quickly scratch themselves against the gravel or aquarium decorations. The life cycle of the parasite at tropical temperatures lasts about 4 days and in coldwater temperatures (less than 55°F) it will be more like six weeks. Therefore, if the tank is left without fish in it running for at least a week at temperatures over 70°F, the parasites should die off. Treatment in marine aquariums is difficult because most chemicals to treat white spot contain copper which can kill invertebrates. Isolation to a treatment tank is recommended to prevent this.
  • Cotton-Wool Disease: Cotton-wool disease, otherwise known as "mouth fungus," generally manifests itself as white, cotton-like growths around the mouth of the fish. It often joined by the appearance of red sores on the body. The most common abnormal behavior associated with this disease is "shimmy," where the fish will hover in one place in the aquarium, kind of wobbling back in forth in an exaggerated swimming motion but not going anywhere. This illness is caused by Flavobacterium bacteria and is not a true fungal infection as is often thought. Treatment of the disease is usually successful when caught in the early stages by using an antibacterial remedy.
  • Body Fungus: Body fungus and fungal infections of fish eggs are caused by aquatic fungi such as Achlya and Saprolegnia. Body fungus is rarely reported in marine aquariums and tends to be primarily a freshwater disease. Fungal infections are usually a direct result of excess organic material rotting in aquariums. Fungal spores are often present in nearly every aquatic environment, but fish tend to resist infection because of the protective slime coat on their bodies that acts like a shield against the spores. However, if the fish is stressed in some way due to rough handling, fighting with other fish, or already has an open wound or fin damage of some kind, fungal infections become more likely. Indeed, body fungus is often a secondary infection to other diseases like fin rot or hemorrhagic septicemia.
  • Dropsy: Dropsy is caused by various factors such as bacterial infections, nutritional deficiencies and metabolic disorders. What it generally will look like is the body will take on an overall swollen appearance with the scales literally standing out from the body giving the fish the characteristic "pinecone" look associated with this disease. Sometimes the eyes will bulge as they do in cases of pop-eye. The precise causes for this disease are difficult to determine, however, the positive side of it is that it is rarely contagious to other fish. Usually simply by isolating the fish and maintaining as perfect water quality as possible, symptoms will usually go away with time. If they do not, treatment with an antibacterial remedy is recommended.
  • Fin Rot: Fin rot appears in aquarium fish as ragged, torn looking fins with white edges to them. It is caused usually by Flavobacterium bacteria but also by Aeromonas and Pseudomonas bacteria. Since Flavobacterium is a common culprit, fish with fin rot may also experience infections of cotton-wool disease. Fin rot's most common cause is poor water quality and husbandry; fish that have experienced shipping stress are also very susceptible. Overcrowding, overfeeding and high ammonia and nitrite concentrations will often cause fin rot. Generally the use of an antibiotic will treat the disease effectively.
  • Fish lice/Gill maggots: Fice lice (Argulus) and gill maggots (Ergasilus) occur mostly in freshwater outdoor ponds. They are rarely a problem in aquariums although occasionally can be found on fish that have been recently imported. Treatment with organophosphorus insecticides like metriphonate are the most successful. Marine invertebrates, orfe and piranhas are very sensitive to this treatment and should be removed if possible.
  • Pop-Eye: Pop eye, like dropsy, is caused by various factors like poor water quality, bacterial infection or parasite infestation. As would be expected, the fish will have an abnormal bulged appearance to the eye and as with dropsy, usually only one fish is affected and it isn't likely to be contagious. Isolate affected fish and provide ideal water quality and a high quality varied diet. In the worst cases of this disease, the eye will actually burst out of the socket and leave the fish with only one eye. Fluid buildup behind the eyes as a result of parasitic eye flukes or bacterial irritation is what usually causes the "popped" appearance of the eye.
  • Head and Lateral Line Erosion: Also called Hole-in-the-head disease in freshwater fish, Head and Lateral Line Erosion (HLLE) is a disease that is difficult to treat and equally difficult to determine the cause of. Infectious protozoa have been named as causing some cases of the disease but bacterial infections, poor water quality and poor diet are other factors contributing to it. HLLE and Hole-in-the-head will look like small pits and grooves around the head of the fish. In marine fish, HLLE is most commonly seen in surgeonfish from the family Acanthuridae and marine angelfish from the Pomacanthidae family. Hole-in-the-head in freshwater species is most common to cichlids, particularly South American cichlids like oscars, Jack Demseys, discus, and freshwater angelfish. From my own experience, I've seen hole-in-the-head more often in cichlids that are fed only live feeder fish, or with feeder fish being the primary source of nutrition. Without a varied diet consisting of commericial pelleted food mixed with frozen food, insects, and the occasional feeder fish as a treat, hole-in-the-head seems to be much more common. In the case of infestation by Hexamita or other protozoa, usually these infections remain mild and are controllable by the fish's immune system. However, when aggravated by stressful water conditions, infections may become worse and symptoms may become more pronounced.
  • Swimbladder Problems: Swimbladder problems are almost always seen in fancy goldfish and are more rare with other types of fish. The reason for this is the extensive breeding of carp to get the distorted body shapes of features of ryukins, orandas, moors, and others have warped the position and shape of the swimbladder in the fish's body. A fish with swimbladder disorder will periodically float on its side on the water's surface or swim on its side a lot. Usually it will only be periodically, with the fish sometimes able to swim and act normally. If a fish lies on its side on the water's surface without moving at all even after touching it, it may indicate a fatal kidney problem which fancy goldfish are predisposed to. Despite these predispositions, there are things the fishkeeper can do to help prevent the disorder. Goldfish in particular are known to swim greedily to the surface to suck down food, but discourage this by soaking flake food before feeding so that it will sink lower in the water column and goldfish won't take in as much air. Also, the food will be less dry and instead of taking on moisture in the digestive tract and swelling up, causing blockages, the food will be more moist and easily digestible. Offering more vegetable content in the goldfish's diet may also help, as well as the addition of aquarium salt to the water.
  • Velvet Disease: Velvet disease is caused by a parasitic infection by the dinoflagellates Piscinoodinium in freshwater fish and Amyloodinium in marine fish. What it looks like is a very fine coating of small, gold dots all over the fish and is sometimes mistaken for ich. It can also attack only the gills and not be visible anywhere else, in which case the cause of a fish's death is hard to determine. A treatment with an anti-parasite remedy is best; the addition of salt is also usually beneficial in the case of freshwater fish. Treatment with copper for three to four weeks is recommended for marine fish in an isolation tank. Keep the tank as dark as possible to prevent the parasites from being able to photosynthesize and try raising the tank temperature to speed up the parasites' life cycle.

Commonly Used Medications

There are a large number of treatments available for fish diseases. The medications available for bacterial and fungal infections are generally very easy to find in local fish stores. Be sure to examine the packages for various treatments to determine what is in them. Methylene blue is perhaps one of the most well-known medications for fish disease, although it is becoming less popular now. Be careful when using it because it will usually kill freshwater plants, disrupt the nitrogen cycle by killing off populations of Nitrosomonas and Nitrobacter bacteria, and stain the silicone seals on fishtanks. Chlortetracycline and Furazolidone are two other medications common in treating systemic bacterial infections. Some products to look for when shopping at a fish store would be Maracyn, Maracyn II, MarOxy, Melafix and Pimafix.

Treatments for external parasites include formalin and malachite green, two of the more well-known medications available for this purpose. Malachite green should be used with extreme caution as it is known for being a carcinogen. Formalin is a solution of formaldehyde gas in water. Formalin can also be very difficult to use because it has a tendency to be irritating to the eyes and skin. Formalin isn't as popular as it used to be as an anti-parasite remedy, but it can still be found in fish stores. Often times, these two chemicals are combined together since their therapeutic properties seem to be enhanced when used in conjuction with each other. Copper is another common medication, and is often found in several medications used for this purpose in fish stores. A good medication that I tend to think is best for parasitic infections is Coppersafe, manufactured by Mardel; it contains a chelated copper treatment that is a good all-purpose anti-parasite medication.

In Closing

While by no means a fully comprehensive list or explanation of fish diseases, the above should give prospective fishkeepers an idea of what fish disease is all about. Perhaps the most important point to remember is that nearly every illness in fish can be prevented by good husbandry; neglecting water changes, feeding improperly and overcrowding are all necessarily going to lead to fish disease and death at some point. So, do your partial water changes, feed a good varied diet, and your little fishy friends should be just fine. Happy fishkeeping!


Burgess, P. and Bailey, M. A-Z of Tropical Fish Diseases and Health Problems. Ringpress Books Ltd., 1998.
Noga, E.J. Fish Disease: Diagnosis and Treatment. Iowa State University Press, 2000.
Various articles from the magazines Tropical Fish Hobbyist and Freshwater and Marine Aquarium, as well as my own assorted knowledge and experience.

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