Sucks that we have to repeatedly and en masse literally inject animals with acid and observe the effects before we/veterinarians/biologists/society as a whole will believe that those animals can feel pain. I love how this shit is only legal because we don't think fish feel pain - but turns out! They do! Gotcha! Wotcher, 'Arry! Wonder how all those scientists torturing fish to find out if it hurt felt when the answers started coming back: Yes, yes it does. Do you feel like a shit about yourself? Because I would feel like a shit about myself if I was injecting acid into defenseless experimental creatures on the reg. Even if it was in the name of holy Science. Pain, like hunger, strikes me as one of those biological constructs which is probably essential to life - at least, long-term species survival.
Yes, I bet it feels awful conducting those experiments; but consider that if people are to change laws around the treatment of fish, they must be convinced first. As crazy as it sounds, those fish might suffer so that others do less. For any of us that eat fish that we haven’t caught, we ask others to do as much on our behalf.
If it helps at all, ideally speaking, these days experiments are much more controlled to minimize as much animal suffering as possible. The regulations and enforcement aren't flawless, but it's genuine progress.