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
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.