There are lots of fish you CAN do in a 30 gallon. What you have to keep in mind is that once fish start breeding most of them don't stop. You have to have a plan for the fry, be it feeding them to adult fish, selling them, or allowing them to fend for themselves and keeping the survivors you need to know what you'll be doing with the young. If you want to sell them, you need to go for a fish you can sell repeatedly.
Laugh all you will, guppies sell and you can breed them with little difficulty. Many types of barbs etc can be raised in a 30 g but you cant spawn and raise fry in the same tank. Once you know what you want to do with the fry you can pick a species.