Protozoan parasite viz. Ichthyophthirius multifiliis.
Fatal if not treated, but progression of infection is normally slow enough for recognition and succesful treatment. Treatment should eliminate completly.
Ichthyopthirius multifiliis cannot tolerate over 1 ppt salinity & thus salt can be used both as a cure & a preventantive. It is especially useful for euryhaline fishes such as poeciliids, which are prone to develop "ich" when kept in freshwater. Most fishes tolerate low salt levels. Salt also helps to alleviate the osmotic stress caused by the epithelial damage.
DVM Noga, Ed
N.Carolina State Univ. Coll.Vet.Medicine
Dept. of Companion Animal Medicine
Raleigh NC USA