I agree completely with the fact that your most likley going to have to sell more then just fish and fish equipment. We have a privatly owned local pet shop here, and it's obvious that the owners true love is of fish, but she also has a few rabbits, cats, and reptiles for sale to. It's rented out on the bottom floor of Charlottetown mall (PEI) and gets a fair bit of foot traffic, and seems to be realitivley busy. They also carry alot of equipment, which i would say makes up 2 thirds of their revenue.
That being said, back in Moncton theres another privatly owned fish (and a bit o reptile) store that rents out a building with another pet store that sells cats, rabbits, dogs etc. and they do quite well, so if you can set up shop next to another shop like this, the combined stock variety is likley to be benificial to both of you.
Those are two examples of private fish stores I really liked, just to give you and idea of what works.
That being said, I am also studing business in College, and would be happy to help yo out with and business orientated questions you may have. I would think that unless you have substatial capital, or can aquire a loan, your going to most likley have to start selling fish out of your home to start with, especially if you were planning on renting out a area in the city ($$$) Also have you taken care of all the paper work needed to start a business? such as an employer number, registering your business name, etc.
As a final thought, bettas seem to be the smartest choice to attempt to sell, they're quite popular, beautiful, and you won't feel unethical selling them to people since with a few guidelines, they'll be able to take care of them easily (rather than, goldfish for example).