you can breed alot of things in a 30 gallon, just need the right pair and they will do it.
I remember splitting up my 65gallon in half and i had three 5" red belly piranhas in there, two of them bred every 8 days...was really fun to watch...too bad i couldnt rear any fry.
I myself just picked up some convicts to try breeding in a smaller tank, did so because I hear they make great feeders, and convicts can be had insanely cheap, got them 2 for a buck so just picked up 4 for now hopefully it goes well!