I'm sure all of us would love to start a fish store, but in reality, overhead, and inventory can kill you. I run my own home-based business fixing electronics, game consoles and computers. I don't stock any computer parts, why, because they change in price and having 1 of everything in stock kills you when it doesn't sell or it goes down in price by 25% because the next generation of video cards, cpus, etc, come out. I had a retail store and overhead kills. Everything is twice as much for a business phone, business internet, business hydro, business insurance, etc.
Same with cell phone repair or laptop screen repair. To be good at it, you need everything in stock. No one wants to wait a week to get their cell phone fixed while you order a screen, they want it done the same day. Do you have an idea of how many different cell phones there are out there or laptops and how many screens you have to have in stock to keep people happy. Luckily, for those services, I outsource. lol. But I know in order to keep buying more cell phone screens and parts to keep in stock for repair, they have to sell stupid overpriced phone cases and swag in there store.
With a fish store, you're going to have to stock lots of crap that will sit there for a long time just to have it in stock in case someone wants it. Imagine you had 20 filters in stock, Fluval 105, 205, 305, and 405's. Well the 06 line just came out, everyone coming into your store is going to want the 06's, so what do you do with the 05's? Sell them at basically wholesale cost to dump them to even get enough money to buy the 06's.
To get an real discounts, you have to buy big from suppliers and that sometimes means contracts to buy so much, having to buy other products as well, etc.
If there are 4 other stores there, you'll have competition to deal with and more importantly, their customer base. Customer base is everything in business. Starting with a small home based business, get yourself established online and sell from there and see if you can make a profit. If you can't get any customers importing cheap filters and accessories from China and selling them or breeding fish/shrimp at home first, you won't make it with a retail front.
You do have an option to try for a specialty store if you did open one, one that focused on say plants and planted tank stuff, or shrimp or something. This place around me seems to be doing great and they do just that. http://www.aquainspiration.com/
But with that, then comes dealing with less clients because 90% of joe consumer wants a cheap tank with a goldfish or betta to shut their kid up. Today with the internet and cheap prices, while it may be nice to go to a store and buy some nice plants or shrimp or CO2 gear, when you can order online and have it at your door in 2 days for 40% cheaper, most people just order online nowadays. lol.