One thing you could do is sell seeded filter media, for beginners. Instant cycled tank!!!! Yay! And it would be soooo easy to do. And no body's doing it!
Keeping a small stock of snails, will so give you babies to sell, and wouldn't require a large space.
You can easily grow plants to sell. Also.
Stuff like shrimps are popular and interesting and breed well also.
Just a thought