I have a rainbow shark that is about 7 years old. He's still very active and none of the tankmates seem sick, but the shark has large white patches on his scales. It doesn't look like ick, just discoloration and I don't see it on any of the other fish. He's close to the maximum life expectancy for a rainbow shark, so I know he probably won't live much longer, but I don't know if his discoloration is from extreme age or if there's a disease in the tank. Do fish go gray?