Fish changing colour

[Joe549]

I have a goldfish, he's actually more silver than gold, that seems to change colour now and then. It has on several times turned black, the last time it happened was shortly after I put some Nutrafin Clear Fast into the tank. The fish is returning to his normal colour now, but this usually lasts a couple of weeks when he changes colour before he returns to normal. This does not happen to any of the other 5 goldfish. He does not seem to be effected by this change, still swims around like always, eats when feed..... Has anyone ever heard of this or seen it happen??