Fish being in 'pain' is a huge subject that science has yet to thoroughly delve into. So is the concept of 'suffering.' People tend to 'personify' the fish. The human conditon of Anthropomorphism is something that animal researchers constantly have to deal with and, watch out for. (Yes. It's worth looking them up in your dictionary).
I will say this -- a fish under stress (which would include pain and discomfort) would usually stop eating. I'm not convinced this fish, or others that have been similarly infected, are in fact in pain or suffering. This doesn't mean euthanizing this fish isn't the correct coarse. Other than pain and discomfort, there are other legitimate reasons for euthanizing, given in that post.