i have a gold sevrum about 4-5 inches in a 75gal and a 3in rainbow shark in a 30gal. bolth of them seem to be losing there color. the sevrum was a dark yellow now its only a hint of yellow and he hides all day under things where its dark, and only comes out to eat. The rainbow shark was a rich dark black but now he's more of a dark meidum gray. his behavior seems fine, but i hered the darker they are the healthier they are. im mostly worried about the sevrum i cant really think of wht it could be theres no marks or cysts or anything on him none of the other fish bother him the only thing i could really think of is that it mite be a parasite of some sort, but i dont really know much obout parasites so i could be wrong.