I always found that their ballasts always used to go, so they'd end up lying on their sides on the bottom of the tank and then struggle around for a few days. It would be too painful to watch.

I rested the net on top of one of them for ages, trying to pin it to the bottom of the tank, so it couldn't move and would suffocate quickly. I left a towel over the bowl so I didn't have to watch and left it there for ages, then I went to check on it, move the net and it shot up and swam round for a wild 10 seconds and went back to struggling. That was the end of my fish keeping days.