I just wanted to say that if you are running a business out of home you have to be careful as I think it can be illegal or require permits etc. in some places.
I know I have seen a couple of A Current Affair type shows where people have gotten in trouble for running a full-blown business from their garage. Definitely something I would look into with the local council before I went ahead.
Also, do you need any kind of insurance if members of the general public are coming onto your property or into your shop? Insurance can be costly, but the last thing you want is for someone to slip on a wet patch of floor and take you to court.
The best thing you could probably do is to get in touch with the big wholesalers for your country. We have several here in Australia that I know of. Two supply livestock, frozen foods and equipment, while the other one supplies mainly aquascaping wood and rock.
I think you are probably going to have to look at purchasing from wholesalers as well as breeding and selling your own stock. People really want to be able to go to a store/business and purchase not just a fish but a whole set-up. Also I believe in fish stores, the money is not actually in the livestock, but is actually in the dry goods part of things.
I would possibly look into running an online side of the business as well. We have a lady here who imports show quality bettas and other 'niche' market fish into the country. She has a brick and mortar store, but also has a channel on Youtube where people from other states (such as me) can order livestock and other things through her and get them shipped out. I have spent probably over $1000 this year just on bettas at her store so it can be quite lucrative.
I would really encourage you to read up and research as much as you possibly can on starting a small business. If you can't do a business based diploma or degree, you really need to hit the books and the internet and draw up a detailed business plan. There is a reason many small business fail in their first year as it can be incredibly challenging, and if you are the only one working there and it is your main source of income you can get very burned out from a 6-7 day workload.