Aquarium fish are susceptible to infectious and non-infectious diseases. Infectious diseases are caused by biological agents such as bacteria or protozoas that attack susceptible fish, reproduce, and are then transmitted to other fish. Non-infectious diseases are nontransmissible and can be caused by a variety of factors, such as poor nutrition or poor water quality.
Many diseases that are unknown in fish in the natural habitat can occur in aquariums under circumstances that favour the disease development. The immune system of healthy aquatic animals protects them from invasion by disease agents. A minor infection can progress to clinical disease only when the balance between the disease agent and the fish or invertebrate shifts in favour of the disease agent.
The outbreak of disease in an aquarium is serious, especially for the new hobbyist who is generally inexperienced in recognition and control of fish diseases. The best approach to control is always prevention-by providing proper environmental conditions, a good program of aquarium maintenance, a sound diet, and by the use of other preventive measures.
To prevent disease outbreaks, it is important to understand how diseases occur. Disease is a process initiated by reduced resistance of the fish. It is a disruption of the delicate balance involving the fish, the environmental conditions, and the disease agents. Research to date strongly shows that the disease process can be initiated by exposure to unfavourable environmental changes such as temperature fluctuations, persistent ammonia concentrations, low oxygen levels, overcrowding, and inadequate diets. Each of these factors can play a role in the initiation and development of a disease. The conditions listed above are referred to as stress factors. These stress factors singly or in combination cause the stress response, which, depending on the intensity and duration can be either beneficial or potentially hazardous to the fish.