This one sucks because I think it isn’t unknown, but easy to ignore. The worst thing I’ve seen related to this was the other day. Someone posted a video of a barn during ventilation shutdown for culling pigs and it was one of the most barbaric things I’ve seen. It was like something from a death camp.
While some say the debate is not settled, I believe there is ample evidence to say that fish do indeed feel pain. You can find literature in the source above.