I have a fairly large black moor (yes goldfish). Thing is from time to time, he gets kind of listless, normally swims in one spot or actively around the tank. but some times he just chills on the bottom, an almost doesn't move except during feeding time. He kind of changes color or tone, and if you look closely it almost looks like he's "dry".. I've looked in my books, and done some looking around online, but nothing firm yet. any sugguestions on what this may be? the cause? the fix? and to make it even harder, none of the other fish in the tank ever have this problem...