Costiasis is an illness which is almost fatal.
By the time it could be diagnosed it is too late for treatment, and also the fish is greatest destroyed to steer clear of spreading the infection among the other fish.
Costiasis is recognized by a reddish streak, not unlike a burn mark, spreading along the side of the fish from the tail towards the head. This streak causes all of the mucus to disappear, and also the scales to appear rough and inflamed.