Here is what i have been thinking about as adds:

1) set the begining up like rollercoaster tycoon where you can pic what location you want, then you set up the store depending on space available (number of tanks, Etc...)

2) Deffinitly fish counter, its a must...just too hard to count 'em all.

3)More fish types

4) I was thinking what about orders you have customers come in that place orders and you have to try to breed the fish before the time limit is up.. ( idk i think it would be fun )