I was just trying to brainstorm and figure out what would cause the wrasse to gulp for air, as I've never experienced that before. I just looked back over my notes, and I did give 3 fish a H2O2 bath simultaneously without issue. So maybe, as you suggested, it was just a wrasse being a wrasse.I used tank water for it so I would think it would be oxygenated enough? The other two fish were not breathing heavy though which is why I didn't panic.
I totally agree the container is small. I pretty much just grabbed what I had on hand.