I don't see any issues with its swimming. I did a little research and this problem may have been a "bloat" problem. I have been feeding them flake food for most of the time I had them, and then gave them a black worm treat a few days before. Then I feed them flake again. I don't reminder if they had one or two feedings of flake before this problem showed up.
Does this sound plausible to you? “It was a bloat issue caused by the switch in foods. This problem cleared up on its own through normal digestion. That is why they seem fine now.”
The fish did look like it was in distress.
The future is what you plan for, life is what you get, enjoy it.