Well, I would say that they probably -do- feel pain. Pain is nature's way of saying something is wrong. If we were talking about a virus or a bacteria, I'd be inclined to say they don't feel it anywhere near how humsn feel it.
However, fish have all 5 senses, taste, touch, smell(?), hearing and sight. While the Korea story is incredible, I doubt nature would take a proven mechanism (pain) and make it a lower order of magnitude of 'something being wrong'. Part of the decision making in the 'fight or flight' process is determining how much pain one might take from a confrontation, no?
Kind of hard to do that without some sort of feedback mechanism, which pain solves just fine.