I have been thinking about this myself, once.
Knowing a lot about keeping fishes would seem to be the first, but the thing is, you will need clients.
Once you have these, it is easy to know what they want. Do they want nice electricity? Then it is something you will have to find out. Firstly you will buy everything from a petshop, later the amounds you'll buy will enable you to go to a wholeseller
But I think the main issue would be to have clients. That is, marketing.
