Fungal, Bacterial, and Viral Diseases

Funguses, bacteria, and viruses are responsible for various diseases of aquarium fish. Collectively, this group of disease agents are referred to as microbes. All are so small that it is not possible to see the organism without the use of a microscope. Identification of diseases caused by these organisms must initially rely on external signs such as lesions or nodules.
Microbial disease outbreaks in aquariums are correlated with a deterioration of water quality, parasites that damage the fish's skin or gills, or poor nutrition. Any trauma that weakens the fish's disease resistance barriers allows invasion by microbes. Microbial diseases are readily transmitted from one fish to another.
Fungal Infections: Fungal infections are not uncommon in aquarium fish. Infection often follows prior damage to a fish, caused by handling with a net or by an existing infection by parasites.
Aquarium hobbyists familiar with fungal infections of freshwater fishes will be familiar with the characteristic white, cottonlike growth of fungus on the skin or fins of infected fish. Known as Saprolegnia sp., this fungus readily invades stressed freshwater fish, but is not known to affect decorative fishes. Signs of fungal infection in fish are therefore less dramatic and often difficult to distinguish.
When present, the fungus appears on fishes as a fine film, dark pigmented areas, or a coating covering areas of the body or gills. This coating can easily be confused with similar signs that are caused by the presence of various types of parasites.
Treatment: Treatment of fungal infections of the body and gills involves the use of various fungicides available commercially. Malachite green and methylene blue have been used successfully to control fungal infections in decorative fish. Various drugs are also useful for controlling both fungal and bacterial infections. Since fungus is a secondary invader, treatment methods must also address the initial cause of the disease, including trauma induced by deteriorated water quality, poor nutrition, or poor handling of the fish.

Bacterial infections: These are also commonly associated with preexisting trauma or stress of fish, including poor water quality, handling trauma, and poor nutrition. Pathogenic bacteria-those that can cause disease-as well as the beneficial nitrifying bacteria are always present in aquarium water. The pathogenic bacteria will initiate disease only if the fish's immune system is impaired. Providing optimal water quality, controlling the presence of parasites, and practising good aquarium management will prevent the outbreak of bacterial diseases. The exact identification of bacteria requires the use of sophisticated diagnostic tests routinely used by laboratory technicians. The procedures involve growing the bacteria on specialised media, then conducting various chemical tests.
Many bacterial infections can be identified by various signs they produce when they infect fish. Bacterial infections can be divided into those causing external infections and those primarily causing internal lesions. It must be noted that external infections can rapidly worsen, spreading to the internal organs and bloodstream. Bacteria can enter fish either through the skin or by oral ingestion.
External bacterial infections (fin-and-tail rot and ulcer disease) can be caused by various species of bacteria. Bacteria most commonly associated with these infections include pseudomonas, myxobacteria, and vibrio. The latter group is the major cause of bacterial infections, especially in newly imported fish that are still under extreme stress from capture and transport.
Collectively, the signs of external infections caused by these bacteria are often referred to as "fin-and-tail rot" and "ulcer" disease. The names refer to the signs of infections from bacterial disease and not to their cause from specific bacteria.
Treatment: Bacterial infections must be treated promptly because of the rapid progression of the disease from minor to severe. Various antimicrobials are available for treating bacterial diseases of fish. Those recommended for the treatment of external infections of the body surface include nitrofurazone, nifurpirinol, and various sulfonamides. Although various antimicrobials are available commercially for the treatment of bacterial infections, not all are suitable for use in freshwater aquariums. Those that are virtually useless for treatment of infections in aquariums include ampicillin, penicillin, tetracycline, and erythromycin. The latter can kill the nitrifying bacteria in your aquarium, causing an increase in toxic ammonia. In addition to treating your aquarium, you should also investigate other possible causes of the outbreak, including the addition of non-quarantined fish, excessively high water temperatures, an overcrowded aquarium, low-dissolved oxygen, or other parameters that could have initiated the infection.
The signs of the disease differ with the species of fish as well as the extent of infection. Typically, fish can develop a pronounced swelling of the body with a change in normal body coloration. The appearance of signs associated with mycobacterio-sis is largely dependent on the stage of the disease. Fish in advanced stages become emaciated, have swollen or cloudy eyes, and can develop ulcers on the body.

Viruses: Decorative fish are also susceptible to infection by viruses, although few have been identified from aquarium fish. Viruses are smaller than bacteria and require the use of sophisticated electron microscopes for their identification.
The major virus common among freshwater fishes and relatively easy to identify from the lesions it produces is lymphocystis. The disease is chronic, meaning that in the majority of cases it will not kill the infected fish. The major consequence of lymphocystis disease is disfigurement of the fish.
The disease agent preferentially infects the cells of the skin and fins, causing the appearance of lesions. Once the virus infects a cell, it takes over the activities of the cell, forcing it to manufacture more viruses.
Treatment: There is no known cure for lymphocystis. The use of medications including antibiotics appears to be useless. If the lesions are restricted to the distal area of a fin, it is possible to carefully trim off the infected portion and treat with an antibiotic to prevent a bacterial Infection. In many cases, however, the virus will reappear in the same area.