Opponents to consciousness in fish argue that fish do not feel pain because they do not have a neocortex, which is a necessary condition for feeling pain. A common counter-argument appeals to the multiple realizability of pain: while a neocortex might be necessary for feeling pain in humans, pain might be realized differently in fish. This paper argues, first, that it is impossible to find a criterion allowing us to demarcate between plausible and implausible cases of multiple realization of pain without running into a circular argument. Second, opponents to consciousness in fish cannot be provided with reasons to believe in the multiple realizability of pain. I conclude that the debate on the existence of pain in fish is impossible to settle by relying on the multiple realization argument.