Maintain tanks. This is hard work, though.
Selling frags or being a basement shop is fine when the economy is booming, but the last downturn flooded the market with frag tanks and equipment at 20 cents on the dollar from people going belly up (even some really nice shops)... this is a hard business and buyers dry up when the money does. Also, do not try and figure out your hourly rate.
When selling to people, you are operating with the least common denominator since every transaction has a hobbyist on the other side... people will kill stuff, blame you... get ich, blame you. You can ignore them since you rightfully probably had nothing to do with this, get a bad rep and then business suffers. You can cater to them, keep everybody happy and make little-to-no money. Did I mention that this is a hard business?
You can also expect to become a semi-professional social media expert... but not in a fun way.
You might have some good times, but expect some bad as well.
It is one thing to grow your own coral and sell it, but you better have huge colonies of stuff. Most people need to import wild/mari stuff and use some throughput to be in the black.
You might better off just delivering Pizzas... not in the business of love, but it is a legit way to make money that is sustainable. I do not want to discourage anybody, but people usually have a more grand view of this than what reality offers.