1 of my 4 sharks has had most of its barbels wither away. No growths of any kind visible. He's not eating, probably because he can't sense or taste anything without his whiskers, where the taste buds are. He just swims around near the top, which is unusual, considering he must be low on energy. The other 3 are perfect, and they hang out mostly on the bottom. I've done a water change. Any ideas?