My fish feel remorse, jealousy, contempt, and claustrophobic but I don't think they feel pain as we do. Of course I could be wrong and I hope I am correct because virtually all fish in the sea die by being eaten while still alive or suffocating in the hold of a ship. Fish in my tank die of old age but that is a very rare thing for a fish.
I didn't come to this decision overnight. It came from almost 50 years of SCUBA diving with fish and keeping them longer than that.
I have seen a shark that another shark tore out his guts and his intestines were trailing behind him. The injured shark only seemed interested in eating his own intestines. If he felt what we think is pain, I don't know but if it was me, I would be more interested in yelling OMG, WHAT AM I GOING TO DO NOW.
Another example is when I kept a hippo tang and I also had a triggerfish. The tang was eating a worm and the trigger grabbed the worm and tore off the tang's jaw. The tang didn't seem to notice and kept trying to eat. He lived for a few days and acted quite normally.
One more example is if you have ever gone flounder fishing and caught a flounder that was too small so you removed the hook, and maybe ripped half his mouth off, then threw the fish back only to catch the same fish five minutes later.
I can't read a fishes mind, but I really hope they can't feel pain. I know they feel something, just not really pain. I hope anyway.