I've been noticing my 8 inch common goldfish has been sitting on the bottom a lot lately. His appetite is normal, greets me like always. Today my fiance and I happened to look at the tank at the same time and he was pooping and what looked like farting... he was losing a lot of gases from the same hole, then immediately he went to the bottom. Giving him a closer look it seems like he has to "work" to stay up. Should I be concerned any ideas what's going on here? Trusty Google said fish don't fart.