Ok, listen here sonny.

I think you gotta define pain first of all. I'm defining it as the human characteristic of disabling discomfort, I think we all know the feeling of pain, so I won't explain it in depth. Fish do not have the neurological system, or the brain power to experience pain like we do. Not even a fraction of it. I do think that fish know when something isn't right, or something is happening that's going to prevent them from functioning (eating, reproducing, keep living...) and that's what they're reacting to. People like PETA, and extreme radical environmentalists like rob want to use the word 'pain', but it doesn't apply. People and fish have two COMPLETELY different anatomies, brain functions, nerve systems, chemical balances, etc etc. Fish probably experience feelings that we don't even have words for. The closest thing to pain that fish are able to comprehend is that something is imminently wrong and needs to be fixed asap so that they can go back to eating and reproducing.
-Brian