Over the course of my life I have thoroughly sucked all the joy out of a couple of former hobbies by trying to make money at them!
Not interested in doing that anymore; fun is fun, work is work, and never the twain shall meet...
But if you are one of those incredibly lucky (...and rare!) people who manage to make money doing what they love...and yet still manage to love doing it...more power to you.
I don't actually expect to turn a profit selling fish that I have bred, but usually manage to produce enough to pay for supplies and occasional new fish...either by trading them in for credit at a trusted LFS or by utilizing the end-of-meeting auction at the local aquarium club. Since the club has had its meetings suspended for over a year now thanks to Covid, things have been slow. At either of those venues, the key for me has been to produce something which people actually want (duh...!) and which isn't typically available, or available only sporadically. Cichlids, plecos, oddballs, Corydoras, etc. have all done well for me. I usually get bored with a species after awhile and move on to something else; I virtually never set out to breed a species, I just try to keep them in ideal conditions and let nature take its course. If eggs and/or fry appear, that's great; I'll capitalize.
At the moment, and for the past few years, I have had decent luck selling/trading Ameca splendens, Xenotoca doadrioi and Skiffia francesae (all Goodeids that I really enjoy keeping, although I'm getting tired of Skiffias...), Red Cherry Shrimp (everybody loves'em and they breed like crazy), and hopefully this coming summer some Gymnogeophagus and Cichlasoma cichlids. Get something you like to keep; that will make the work of keeping them much more palatable.
If you want your parents to accept that you are actually trying to run a business, as opposed to just looking for an excuse to get more tanks and livestock
, you should take into consideration your overhead. Living at home removes the biggest business cost (rent) but you still need to buy equipment, food, supplies, etc. to run your enterprise. You also need to place a value on your time. If you spend 10 hours a week taking care of tanks, rather than getting a part-time job that pays you for your time, that's an expense.
The idea of complementary breeding sounds cool, but finding two species that will cohabitate and breed without conflict, and which are also profitable and can be produced in a relatively small set-up is a lot tougher than just choosing one.
Good luck! Remember to keep it fun!