This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
Probably 80-90% of diseases in captive fish can be prevented by
avoiding stress. Stress weakens fishes' immune systems, leading to
increased susceptibility to disease. Actually, diseases and pathogens
are almost always present in tanks, but a healthy fish's immune system
will prevent them from being a problem. Some of the most common
stressors for captive fish are:
* Poor water quality: measurable ammonia or nitrites, or very high
* The water temperature is fluctuating more than 2 deg F/day
* Incompatible species in the tank.
* Too many fish in the tank (5 adult angelfish in 10g tank).
* The tank is too small for the fish (foot long fish in 10g tank).
* The water is too warm or too cold for the species (goldfish vs.
* wrong pH for species (Discus vs. African cichlids)
* pH fluctuations greater than 0.2 units/day.
* Insufficient cover or hiding places present.
* Wrong water hardness for the species (Discus vs. African
* Insufficient oxygen in the water.
* Improper fish nutrition (wrong food, foods not varied).