Fungal, Bacterial, and Viral Diseases
Microorganisms such as fungi, bacteria, and viruses are present in most aquariums but are invisible to the naked eye. Many of these only rarely attack animals directly. They are opportunistic in nature and become established following some other condition that paves their way. Such conditions may be a wound or other injury, a parasitic infestation or a general decline in the immune system due to poor environmental conditions.
Fungal infections most commonly manifest themselves as a fuzzy coating on the afflicted animal's body. In some species of fish, discolored patches, usually darker than the fish's normal color, may also appear. Fungi of one type or another are present in nearly all marine and freshwater aquariums and generally cause problems only after a fish's or invertebrate's immune system has been compromised.
A wide variety of fungicides are available for treating fungal infestations. Most contain malachite green or methylene blue and, being fairly broad spectrum in nature, will kill a number of troublesome species.
Bacteria, fungi, and viruses frequently afflict aquarium animals during or immediately following parasitic infestations. Therefore, when treating for parasites, you should be particularly alert for symptoms of other related conditions. Often, a second medication may be required to combat opportunistic pathogens.
While certain bacteria directly cause disease, the vast majority manifest themselves after injury or parasitic infection has weakened the infected animal. Bacteria often cause skin ulcerations and the loss of fin and tail tissue.
Bacterial infections are most effectively treated with antibiotics. Unfortunately, most antibiotics will rapidly decimate the population of nitrifying bacteria in the filter. This will result in a sudden rise in ammonia levels, a condition that can weaken and eventually kill even healthy animals.
The viruses that affect fish and invertebrates are, at this point, largely incurable. Viral infections may be diagnosed by the presence of hard white nodules on the body. Other symptoms will vary widely according to the internal organs that are affected by the virus.
A good deal of research is being conducted on the treatment of the viruses that affect commercially valuable fish and invertebrates, such as salmon and lobsters. Hopefully, knowledge gained in these areas will someday translate into medications suitable for treating viral infections in home aquariums.
If opportunistic microorganisms such as viruses seem to be affecting your fish, check for other diseases as well because virues and other diseases usually go hand-in-hand. Fungi, bacteria, and viruses also take hold when animals are stressed by environmental conditions, so be sure to monitor temperature, pH, and ammonia carefully.