Fish magazines, I love magazines, but I can't get any fish ones out here (I live in a small town with a population of around 1,500), could probably get them if I ordered them tho, but I like to have a bit of a flick before I buy them, if I order it I'm obligated to buy it.
No Betta Bowls, god I hate them, and no Betta's in those horrid cups.
Make sure you have every water test kit, again I can't get a Nitrate test kit unless I order it, and then it's going to cost heaps.
You know maybe you could keep tabs on what people buy (maybe by asking them for an email address for your "newsletter" or by name so that you can make sure you stock their type of "fish food"). i.e. they come in and buy a tank (week 1) you keep the stats on what they get and let's say they get a 10lt (only big enough for a Betta) then they do the right thing and cycle it but come in and want an Angel for example, you'll know that the Angel can't possibly fit ........................ Or they come back two days later, claiming the tanks cycled and want the Angel, again you'll know that that can't possible work.
Please don't sell non-aquatic plants ie purple waffle, aluminum plant, that only leads to customer disapointment and the possibility that they wont return.
Love the idea of the buffet table gravel etc. Don't forget background pictures, I don't like them myself but plenty of people do. I like basic black, maybe you could have rolls of plain coloured backgrounds, black, green, blue etc.