I think that depends on how tall the canopy is and the type of fish. Most of the time I think it would be fine, unless the fish in question can swim fast enough to jump over the canopy. I can't see most fish being able to jump more than a 6-8 inches, but I can't say I've ever seen them try to jump, so that's just a guess.
Not sure about the evaporation. I don't think it would be a problem, as there is air flow through the canopy so the humidity wouldn't be that much higher than the air in the house. I would be conserned about splashing and salt creep though.