In any aquarium it is normal to have a low level of disease causing pathogens and under most conditions those pathogens do not cause ill health since the fishes immune system is able to continually fend of any attacks. In the enclosed environment of the aquarium however, some disease pathogens can rapidly multiply and infect fishes in close proximity. When there is a large concentration of a pathogen in the aquarium even healthy well established fish can become ill.
Introduction of ill fishIntroducing fish which are already under the influence of disease is naturally a potential threat to other fishes. The problem is made worse by the stressful experience of being transported and moved into a new environment. This added stress often causes the diseased fish to rapidly deteriorate, and at the same time allows the pathogen to multiply very quickly. If other fish were introduced at the same time, they will also be weakened by the stress of moving and prone to infection, allowing the pathogen to multiply further and affect otherwise healthy fish.
Introduction of unknown pathogensLike other animals, including humans, a fishes immune system is developed by constant exposure to disease pathogens. At low levels, this exposure allows the fishes body to adapt a defence so that any future encounters with the pathogen can be quickly dealt with. If an adult fish happens to encounter high levels of an unfamiliar disease pathogen, it will have little natural defence and is often quickly overcome by disease, allowing the pathogen to multiply. This situation can occur when fishes from different regions or suppliers are placed together in the same aquarium. In this instance, one type of fish may be carrying pathogens that do not affect it, whilst the other fish, which has never encountered the pathogen, may become ill.
Build up of debrismany bacterial and fungal disease pathogens can thrive in waste matter such as leftover food or excess fish waste. When this happens the fish in the aquarium have to deal with much greater numbers of these pathogens than normal, and are more likely to become diseased.