In most new buisneses, you will make a loss the first year, aim to break even the next, and make a profit the third.
From what i have gathered, most lfs make the most profit on the things like tanks and tank eqipment, while the fish hardly make any profit. Plus fish can be very unpredictable- you may lose half of them to a bad shipment, or half of them may come in with whitespot or get it days after settling in, so you could lose vast quantities of fish before you get to even sell any.
Marine fish from what i have heard are relatively profitable, but then again marine tanks are difficult to mature or get stable for the fish particually in a lfs enviroment when the stocking levels are changing all the time and the risk or getting desease is high.
You will also need to attend to your areas needs- in a large town or city, you will face more competeticion, and people will want more different and exciting fish. There may already be a resident large company lfs or two in there making it far too difficult to start up your own first time shop.
You also have to consider the type of people living in the area- fish keeping in general, is an expensive hobby if you want to do it right, and from my observations most people that are in fish keeping are middle class with average or higher than average incomes.
In a small town though, where most people go to the petshop for things like buying food for their furry pets, you won't be able to sell £30 fish very easily.
I dunno...
Why do want to own a lfs? To start up your own or buy a current one? Do you want to breed fish etc?