I don't think emotion has anything to do with intelligence, no matter what scientific studies say. I mean, there are people who have the same brain power as fish (if not less) and we wouldn't keep them in lesser living conditions just because they can't show happines. Emotion is a complex thing, and I'm confident that it's not limited to humans.
Just because something can't show happiness by laughing or smiling or other personified means, doesn't mean it can't feel it. and just like other animals show happiness (purring, tail wagging, etcetera, fish do, too: vibrant colors, more activity, the way they interact with other fish...)
But this is for another thread.