Fish react with a formation of mucous in diverse situations, like e.g. sudden and severe changes in water conditions (like salt levels). However, fish also react to ectoparasites, which settle in the skin, and gill worms by an increased production of mucous. In the case of the latter, the increased mucous production is limited to the gill epithelium (surface of the gill filaments). The fish increases the production of mucous in an effort to get rid of the worms that are anchored in the epithelium, which, though does not succeed. As a result of the increased mucous production, the transport of oxygen via the gills is limited and the animals react (despite sufficient available oxygen - check the oxygen content) with a higher respiratory rate.

Both potential diseases can only be diagnosed with certainty by a smear and a microscopic examination.