I agree that given proper space to workout and good conditions would keep them happy.
In a fish tank, they don't have to worry that some fish would kill them (if the owner is careful), they don't have to worry that they've been doomed from the beginning if they were born an albino or whatever color that would give them away when hiding from predators, they don't have to search high and low for a mere mosquito larvae and die of hunger if there's nothing to be found and don't need to worry about low temperatures and such.
It appears like most fish only care about food and mating. Usually, livebearer males seem to be more interested in the latter.
Some will spend their time exploring or watching people walk around the room or interacting with them (usually, these are the fish that don't really like company of their own kind - bettas for example). Those are interesting, as they can recognize people by faces, and they themselves seem to have different faces too.
But what exactly shows that a fish is happy though?
Is it their healthy aspect and display of natural behavior?
Sometimes fish do glass surfing, are they bored when they do that? Or is it just their way of wondering "Hey, why can't I go there and check that place out?"