Fish DO feel pain. They have pain receptors and a central nervous system. The point is that they do not conceptualize pain and develop an emotional response to it the way higher vertebrates like mammals do. In my opinion your piranhas were simply responding to negative stimuli, and avoiding it. They probably were not in fear to the same extent that your Convicts were not in love with each other--simply doing what biology told them to do for survival (I say probably because the widely held understanding of how the vertebrate brain works certainly could be wrong.)
I read about another study some time ago where the researchers scanned fish for brain activity, then inflicted a painful stimulus. The only place that showed any change in activity was the brain stem, indicating the only response was fight of flight. They weren't tortured--at least not in a way that I would define torture. I looked for information on this study, but came up empty.
So we stick a hook in a fishes mouth, remove it from the water, remove the hook and release it quickly. It is stressed, and its mouth is probably sore, but it isn't afraid, and freaking out, and saying "Why me? What did I do to deserve this?"
I should also mention, however, that when the same scientific community that makes these claims about fish tells me that my little English Setter does not feel guilt and does not love me--I call BS. I cannot watch her behavior and see anything but guilt when she does something wrong and love when she looks up at me when I scratch her ears. So maybe I am entirely wrong.