Rare isn't a good way to go, as the need for a constant supply isn't very good. Inverts, and plants are two of the biggest sellers in many stores. I do $600 a year in snails now, and $1000 in plants. $100 a month isn't very hard at all. If you add on to that something else that is always in huge demand, such as bristlenose plecos, $300 would be very plausible to make each month.