how do you know fish don't feel pain, are you a fish? you have nothing to back that up, imho all living things feel pain, animals like your pets, dogs, cats etc. how is a fish different? Just because we don't know what a fish in pain looks like dosn't mean its not hurting, think about it
Fish's brains are not capable of feeling "pain." There have been studies published on this (that I'v read through school - I can try and find them again in you really need it).
Your "opinion" in that case doesn't matter. If you can come up with scientific backing for your statement, then feel free to tell me I'm wrong.
There have been multiple studies where fish have shown a reaction to something (such as bee venom), but that is most likely due to handling, etc (if it wasn't, why would fish eat bee's, crayfish, spiders, etc). They are more than capable of seeing, so anything you do to them will freak them out and get a reaction.
Try simply ripping a fish's gills out one day (sound brutal, I know). You would think if they felt pain, they would freak out if someone just ripped their lungs out. They don't, they just lie there and seem to calm down as soon as you take your hands off them.