I'd wonder if the store would let you create an "information center". You could put flyers of fish in and fishless cycling and why it is important. Maybe have some flyers on species people commonly buy (like goldfish and bala sharks) that will get too big for 10 gallon aquariums. Have a flyer about tank maintenance, and one on how to treat things like Ich and other things of that sort. Like the other comment said, do some research on what fish the store sells so you know their requirents. I know most people say that a fish store will sell you anything because you will just come back and buy more stuff when all your fish die because you have no clue what you are doing and that's good for business, but I think if a fish store has knowledgable employees that can inform customers on what their fish needs then that customer will certainly come back for the things they want to buy.
I'm sorry, but you will probably sell many betta's along with those betta bowls and tiny box aquariums.
On the plus side- store discounts on fish supplies! (I hope!!)