If you have a good sense of fish keeping and aquarium upkeep, get yourself connected with a local store or 2 as their maintainence person. If you don't know all the fish, their habits (temperment, compatabiliity, etc) and requirements, or have a full knowledge of aquarium systems and how they work, I suggest you try to get a job at a LFS
( Mom & Pop type, not Big Box type) and learn them before you try to be on your own. The fastest way out of the fish biz is to not know what you are doing. That will cost you credibility, customers and cash. Most maintainence people don' t make it past their first tank wipeout with a customer. The customer will usually tell everyone he/she can about your ineptness. That doesn't bode well for the future. I spent over 40 years in the fish biz. I believe I've heard it all
Hope this helps.