While this forum may be a good place to find information on keeping and raising fish I don't think it's the best place to look for advice on starting a business. First of all get in touch with local organizations that can help you learn about the ins and outs of business in general. Second, speak to a lawyer who knows about livestock and make sure you know all of the applicable rules. There may be some stipulations that you wont like and that may dissuade you from opening a fish biz, best to know before you spend much $. After that, set a target for yourself, eg. I want to be Big Al's (Canada's biggest fish retailer [I think anyways]) and see what they are doing that is successful. If you want to do something new, still check out some successful fish stores and see what you can adopt. More importantly, see if the business owner can give you any tips.
Having watched my parents start and operate two seperate businesses and now being groomed to take over one I have learned that there is A LOT more to it than just putting up an open for business sign. If you aren't young and able to start again when you fail, or rich enough already not to go into ruin if you fail you probably want to think it over again. Look up the stats on how many small businesses succeed, or even how many are around after their first 2 or 3 years, you may reconsider after that too.
To add even more problems, you're thinking of a business that, even if its only a small location, will require significant startup costs. As mentioned, there are lisences, there are probably fees associated with those that you must pay yearly. You have to invest in all the tanks, the fish, chemicals, etc etc. You then have to set the damn thing up which will cost you some and if you want to boost sales you're going to want to make it aesthetically appealing as well. Again, if you're not some rich guy already, are you comfortable knowing that the bank will own your ass for as many years as it takes you to pay them back, or worse, indefinetely if you cannot?
All that said, more power to you if you decide to go for it and I wish you the best of luck. There is nothing comparable to being your own boss, and nothing more rewarding than seeing the fruits of your labor. When you've sorted out all of the 'business end' come back here for stocking advice and tips on caring for the fish in your store, the people here are great for that!