Another thing you should include in your store is a good-sized section with well marked aquarium plants. I know for me, I cant find ANY locally other than amazon swords and the common ones, and you have to guess on the species because at my LFS they arent labeled.
If you can cater to both the "betta" end that'll be maybe a parent buying a fish for their kid, and the more exotic freshwater species like rams, ghost knives, eels, etc, geared towards the hobbiest, then you've got yourself a good start.
Stocking all the plant substrates would probably be a good move as well.
I really like the idea of buying and selling used tanks too, maybe you could reseal the old aquariums and test them, then bump up the price on them a bit so you make more profit, and the people buying them know they're getting sturdy tanks.
I think you could stay away from salties for the most part, because most likely the people that are really into saltwater will go to a saltwater specialty store.
Just my ideas - these are most of the things I find lacking at my LFS'