This illness could be recognized by a yellow-brown film which starts near the dorsal fin, and, if left unchecked, it’ll spread all over the body in a velvet-like film.
To treat, dissolve one tablet of Acriflavine (0.46 grain, obtainable from chemists) in eighty drops of hot water, then add five drops of this answer to every gallon of water to be treated.
During this treatment the aquarium should be kept dark, and artificially aerated. Repeat treatment after five days.