Like other captive animals, aquarium fish are vulnerable to a range of diseases, many of them triggered by stress such as overcrowding, excessive noise, aggression from other fish, poor water quality, or changes in temperature or water chemistry. Commonly experienced problems in aquaria include "Ich" or White spot disease, a skin infection caused by the protozoan parasite Ichthyophthirius multifiliis, which manifests as small white spots over the body and fins; ‘fin rot’, where fishes' fins turn whitish and die back, often following damage or injury, which is caused by bacterial or fungal infection; and various internal or external parasites.
The names commonly given to disease problems encountered by ornamental fish hobbyists, such as ‘dropsy’, ‘pop-eye’, ‘fin rot’ and so on, are mostly descriptions of symptoms rather than specific diseases. They may be caused by a wide range of disease agents, most of which are poorly understood. Although some diseases, such as White spot disease, can cause grossly visible signs in fish, the actual cause may not be determined without suitable training and the aid of a microscope, or other specialist diagnostic tests. A veterinarian should therefore be consulted for disease diagnosis, and gross signs alone should not be used to diagnose disease in fish.
The keys to minimising disease problems in ornamental tanks and ponds are to manage the fishes’ environment to minimise stress, to maintain water quality, ensure there is no over-crowding, and to always quarantine live foods and new fish before adding them to the tank.
Finally, great care must be taken in disposing of dead fish, waste water or other materials from fish tanks, as many diseases of ornamental fish can spread into the wild and affect native fish populations. For example, in 2005 Murray cod were found to be highly susceptible to a dwarf gourami iridovirus, an outbreak of which caused 90% losses in farmed Murray cod in Victoria. Similarly, outdoor fish ponds should be carefully sited and built to prevent overflows from reaching natural waterways.
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