I would say that fish has some sort of parasite the head is all sunk in and the belly is sunk in and it has no color.it not an issue for be that he is in a small tank,but I would not post this fish and be proud of it.Do you feed him live food such as goldfish?That could be your problem right there because there is not much nutrition in feeders and plenty of parasites and they use a lot of copper to treat them for any thing they have just to get them to market.And the copper builds up in there internal organs usually winding up with a shorter life span.