Even so, the gills are a weak point which can be exploited by an outbreak of opportunistic organisms. If ever there was a direct physical link between poor water quality and ill health, that link is the gills. Their delicate structure means that they are easily damaged by chemicals such as chlorine, ammonia, formalin and others, as well as by large changes in water pH value. The initial response to chemical attack, or indeed any irritant, is usually hyperplasia, in which the epidermis thickens by producing extra cells. Often this thickening of the epithelium results in the secondary lamellae becoming stuck or clubbed together.
Chronic irritation is also likely to cause a thickening of the mucous layer.
These conditions, together with the stress caused by the accompanying osmoregulatory and respiratory problems, are ideal for opportunist bacteria. If the condition is not treated and resolved it is very likely that secondary opportunistic bacteria, parasites or fungi will gain a foothold. And secondary infections can lead to gill-rot, during which the filaments are slowly eroded away, usually with fatal results. But remember that high levels of parasites are usually brought about by poor water conditions - and therein lies the root of most gill diseases. With a little knowledge it is fairly easy to surmise that there may be a problem.