This will be a little bit of a long story, please bear with me. My boyfriend had a red tail shark in his tank a while ago. One night, it started acting super strange. It couldn't really control its swimming to the point where he would practically launch himself into the sides of the tank or into the gravel, and he was breathing super heavy. I had asked a different forum for help, but all I got was, "put an oxygen pump in it." It's good advice, if he and the other fish had always been gasping for air, but 1) they weren't always gasping for air, and 2) it was literally only the shark. So I got no real help, and he ended up dying. Several weeks go by and my boyfriend and I took our time trying to fix the tank (Still had no idea what actually went wrong, but we kept a close eye on all of the tank parameters for a long time and everything seemed just fine.) Every single fish was doing just fine, so we assumed we either fixed whatever the problem was, or that maybe there was something wrong with the shark itself. Regardless, once we felt comfortable, we got another one. He's been doing just fine for a little over a week, however, when I went to feed them just now, the shark is acting funny. It's not quite as bad as the first one, but he's sorta upright and not moving around. He occasionally darts around for a quick second, then goes back upright. His breathing is super heavy too. Clearly, we didn't fix the issue, and we have absolutely no clue what could be hurting him, and we just really want to help him before he ends up like the first shark did. If anyone knows anything about what's going on, I'd really appreciate any info I can get.