Jon - he was asking if he would need a license.
Jerik - It would depend on how serious you were. If you're just looking to sell fish to stores occasionally, you're not going to need a license. If you're looking to rent a commercial location and start breeding full time, you'd need a business license from the city you're renting the space in.
In order to do this for a living, you're going to need a lot of tanks, experience keeping and breeding fish, startup money, business sense, etc. You'd probably need in excess of 300 aquariums to be able to earn a living, more than that would increase the odds of being successful. You'd need a website at the minimum, learn how to ship fish, and have to attend conventions/trade shows to get your name out there. The likelihood of being successful without ten years of experience keeping and breeding fish is minimal in my opinion.
My advice regarding store credit is, find another store that will give you cash. I've seen too many stories where people were burned by store owners that were unethical or just went out of business.