I have two goldfish in a 20 gallon tank. I've had them for about a month or two, and both have seemed fine up until now. One of them is floating on his side like he's about to die... I thought this was kind of weird, especially because I had just done a water change a couple days ago. Well, after testing the water, I got perfect results (0 ammonia and nitrite, 10 nitrate). Just to be sure I tested again, then got the same results. I noticed some gasping at the surface (definitely not enough to where I would normally start to worry) so I added an air stone just in case. Well, he still looks the same, except he's trying to swim around a little more. I think this is just because of the extra current though. I have no idea what's wrong with him and have no idea what else to do. Can anybody guess what's wrong?