As a result of this response, the normal distance of one cell may be increased to five or six. The oxygen in the water now has several times the distance to travel across the gills before being picked up by the red blood cells. The same is true for the carbon dioxide and ammonia waste products moving across the gills in the opposite direction. This situation may severely compromise the respiratory function of the gills. In addition, the gills contain cells that secrete mucus.
Mucus provides a barrier and alleviates the irritation.
But, the mucus also provides a barrier to the efficient exchange of gasses and ammonia excretion. If the environment of the fish is improved through more efficient biological filtration and water changes, the gill tissue will return to its normal structure. The damage is reversible. Chronically poor water quality, however, will result in continuous stress and finally lead to disease. The problem of the gills producing more mucus or thickening for some reason, is type of a negative spiral situation.