I'm thinking that a good model for a fish store would be one that catered to renters, because most rentals don't allow pets like cats or dogs.
But anyways, which portion of the country would be most ripe for this? For instance, inland areas might like it more because fish are such an exotic departure from their norm of cow fields and such, while coastal areas are pretty used to fish.
Sorry, but you're off on pretty much every count here.
Every city I've lived in, I've not only been able to easily find pet friendly rentals for cats, dogs, rabbits, etc. but I've also never yet had a rental agreement that didn't limit the size of tank I'm allowed. Also, there are rental properties in every part of the country and, I can assure you, even "inland" among the cowfields, folks have heard of that amazing new trend of bringing glass ponds inside and stocking them with live fish.
Every geographical area is going to have advantages and disadvantages. Big cities provide more potential customers, but also higher costs. Coastal areas, especially for reef based stores, offer easy shipping and potentially more folks eager to keep reef tanks--but that very demand means there's already lots of competition set up and running. Very cold regions add hassles with seasonal affect on shipping and sales--but if you take the higher risk you might find yourself the *only* one in the area providing that service. Or you might go out of business (though you'll probably sell a lot of heaters). You'd do better, hypothetically, to figure out approximately where you want to live (mountains, desert, big city or small town, east west etc) and then research that general area's demographics to figure out where the best place to try and set up would be.