Fish can not feel pain.

[ATF]

I'm not convinced that it's productive to relate fish perceptions of pain to human perceptions of pain. It seems as though the scientific literature on the subject is tending towards the idea that fish have instinctive reactions towards threatening situations, something that can be seen in many species. Fish have been roaming the earth for millions upon millions of years as well, and I would wager that they needn't evolve into more complex creatures during this time periods.

tl;dr: Reacting does not indicate pain. Science tends towards less to no pain. Looking forward to more research.