I was thinking the word clientel just before I scrolled down and read tightdog's post. If there is any sort of aquarium group near you that would be a great place to start. If there isn't maybe you could start one. A society would be a great place to find not only customers, but also breeders, employees, and a knowledge base.
I think that more important then marketing and demographics is practicing good fish husbandy. Keep your tanks clean. Have a knowledgable staff. Operate under the principal that if a newcomer has a successful start, they will venture further into the hobby and in doing so remember your business.
My town is about the same size and has a Petco, a PetSmart, two local mom and pop pet stores, one exclusively saltwater, and Walmart. Here are my experiences with each.
PetSmart is the busiest. The tanks are clean and the display is wonderful. They offer returns on fish. They also have the largest selection of products. The help, beyond bagging the fish, is clueless. They will sell you anything you want, but you have to figure the rest out on your own. Most of their fish business comes from I think people who own a cat or dog and then see the fish and get enamored. What I like is that they offer complete setups, complete with a fish credit. What I dont like is the pricing. Perhaps it is just locally here, but their in store products always seem to cost comparably to better brands I can get elsewhere. For example, I bought a Top Fin heater once, and hated the thing. It cost just as much as a Ebo Jager that I bought from the local Mom and Pop. I think it is important to know where equipment can be just good and not exceptional. PetCo is essentially the same.
Of the local mom and pop stores, the pet store has the most knowledable and helpful staff. They will order things, including stock. But, their tanks look bad. Their primary business seems to be birds, reptiles, and ponds. I wouldn't buy a fish that came out of their tanks. I do though buy equipment from them.
The salt water place, as advertised, isn't actually only salt water. They have a few freshwater tanks. They will also special order, and their tanks are clean. Both of these stores seem to float, not by their retailing but by their services. Both offer business and home aquarium services. The tanks that they dservice always look good.
Walmart is a fish cemetery.
One other thing though that I will mention is this. There is a local veterinary supply store that I order though. The owner has mentioned several times wanting to expand his services. He is in the pet keeping field, but carrys no livestock. The staff is very knowledgable and includes a vetrinarian. Maybe you should look for a partner in this venture that is already established but needs someone to help expand successfully.
Anyway, I hope you were able to glean something useful from all of that.