There is a small growth on my fish that resembles a tuft of cotton wool. What is it?

Cotton wool-like growths are usually a sign of a fungal infection. Fungus is a common fish disease that can prove fatal if not treated early. Fungus is usually a secondary infection that follows on from a previous infection or parasite. They are opportunists, and try to infect wounds or abrasions on the fish skin. Fungal infections in fish are usually caused by the genus Saprolegnia. They should be treated as soon as possible to prevent the infection from spreading. King British Fin Rot & Fungus Control, and King British Disease Clear are both effective against common fungal infections (do not use these treatments together).