Well I think the most important.. before adding more and more fish would be to improve the backround.. a 3d backround would make the aquarim much more realist.. also I don't think this would be that hard to add but the ability to select a mp3 to play in the backround of the aquarium...
also it would be nice if the fish didn't swim off the screen.. it's kinda weird since it's trying to emulate a real aquarium.. and um where do the fish go if it's a real aquarium??