I think it's a misconception about many pets; not just fish. Often, many animals that aren't cute/fluffy/cuddly are seen that way, particularly insects/reptiles. This video was quite fascinating though. I'll admit that I didn't think it was possible to train a fish like that, although I think it's possible it was more so his hand gestures than his spoken words that the fish paid attention to.
I think the "fish are stupid" stereotype is often believed because of the very common "fact" (over here at least) that goldfish can't go from one side of a fish bowl to another without losing its memory.
My favourite colour is green, my favourite number is 19.