thea coating of yellow (or brown) on the fish, is called velvet. This is a parasite – a dinoflagellate called Piscinodnium ( i think thats how you spell it ) . The fish may also have clamped fins that scratched against tank decorations or gravel . Once the attack is severe, the fish may become lethargic and you may see rapid gill movements. This parasite will attach to and attack the skin or gills of fish. It is deadly and quite contagious, so treat the whole tank with Rid Ich or Quick Cure. Sometimes velvet can be difficult to diagnose, and it may help to shine a flashlight on the fish to see the yellowish coating.