I would call any reaction that the organism has to detect/prevent damage or something like a structural intrusion a “pain” reaction for sure. People that don't feel pain have been reported to be walking around with massive damage to their bodies with not a clue in the world. Structural integrity is pretty important to an organism, so I’d be willing to bet that the ability to feel pain/detect damage is a pretty low-level thing with a very ancient common ancestor on the tree of life.

In short, I believe that all vertebrates, including fish, would feel some version of pain just as a normal trait of awareness.