Let’s suppose that you believe that a fish can’t feel pain and that I do believe that a fish can feel pain. If we both believe that our moral obligations to others depends on whether or not those others can experience pleasure and pain, then you will believe that you have no moral obligations to that fish. However, if we both believe of ourselves that we don’t want to do what’s bad for others, then since it is clearly bad for all fish to kill them, we both would have a reason not to kill a fish. In that case, what I would need to do, in order to convince you not to kill a fish, is show you how killing fish is bad for them. That seems easy enough to do – a fish can’t be a fish when it’s dead, so killing a fish would be quite bad for a fish as a fish.
Of course, you may think that as bad for a fish as it would be to kill it, there are other things in life that are good for others and that killing a fish provides enough of those goods for others that killing the fish would be something you’d still be willing to do. I’d argue that most of the time we overestimate the good for others that can come from killing fish and that most people don’t understand how bad it would be for a fish, as a fish, to be killed, but those are empirical matters and not moral ones.
As long as the ‘animal question’ is phrased in terms of morality, the question will remain unresolved.