Paul,
Whether or not fish feel pain is currently a hotly debated topic in academic circles. In fact, there was a workshop on fish welfare here on the West Coast a couple of years ago and the consensus was that there was more going within a fish from a sensory perspective than previously thought. Actually, from my perspective pain is not really the issue. It’s the response to stimuli that I am more concerned about. Without a doubt fish respond to stimuli. My main concern when handling fish, especially when administering an injection, is to avoid physically damaging the fish. If a fish struggles while being handled, or reacts suddenly while you've got a needle stuck in its post-orbital space, there is a risk you will cause some significant lacerations. Further, It's my view that handling fish out of water is a major source of stress, which, as I'm sure you know, can have a big impact on immune function. Why take the risk? I don't disagree that fish are pretty tough and can take some pretty rough procedures, it's just not something I would advocate unless I was very confident that the person being advised knew what they were doing. Further, reducing the pressure from fluid build-up solves one problem - but doesn't usually address the root cause, which is often an infection. If you have to repeat the procedure and then administer antibiotics the fish is no better off (using your quick fix analogy). A simpler procedure would be to administer antibiotics via injection, at the onset of Pop-eye development as antibiotics are very efficacious when administered IP or IM and there is less risk of causing damage to the fish. I omitted this from my response as I feel even this procedure is best left to professionals who are properly equipped and trained such as veterinarians.
Keep in mind that I'm not questioning the procedure or your expertise. Rather, I'm just raising a red flag to caution many who may be reading this thread, but have little or no experience handling sick fish, that this procedure may not be as easy as you make it sound.
Cheers,