Tetrahymena 

Tetrahymena ("Guppy Killer Disease") and its marine counterpart Uronema are ciliated protozoans common in crowed conditions and in water with excessive organic debris.  Affected fish may have whitish areas on the body surrounded by hemorrhagic regions.  Diagnosis is confirmed by finding many actively motile pear shaped ciliated parasites on wet mount preparations from affected areas.  This organism can commonly be found on fish that have been dead in the aquarium as they help breakdown organic debris.