I recently got a shubunkin goldfish. he is currently in a tank with 2 other regualr goldfish. He was fine for the first few days. He now seems more lethargic than the others and will often just hang out at the top of the tank and not swim or move around. The other two fish are fine and swimming around normally. He doesn't gasp for air at the top, he just sits there. He looks fine, no torn fins, bloating or discoloration. He will eat when they are fed but the goes back to not swimming. Any ideas what could be wrong with him?