if it looks like gold dust on the fish using the flashlight its velvet. get some anti external parasitic meds. where are you located for specific med (not all anti-parasitics are used for every type of external parasite). also, and may be your easiest method of treatment for now until you get medication, keep the tank dark! throw a blanket/thick towel over the tank. the parasite can not survive without light. keep tank dark for 5 days. do big waterhcange first, feed the fish as regular and cover the tank. keep it covered and tank lights off.
is he flicking.scratching on things? has he lost his appetite? lethargic? just a few questions.
cheers