Personally, I like it as an example here, because yes, technically you can’t know for sure that a fish feels pain, much like you can’t know for sure that a cat feels empathy, but it’s illogical to assume that if we don’t know about it that it doesn’t have it. The base assumption should be that these animals are similar to us, because they’re really not that different from us.
In particular, pain and empathy are crucial to survival for us. It would be extraordinary, if fish and cats survived without any notion of it.