its a difficult one to truly answer, If you speak with people like one of our mod's ludwig venter who has a few tropical ponds, he can be over run with the likes of guppie's and convicts...The problem is you are creating a miniture eco system but forgeting and basically not wanting to add any type of predator, so the only things that may keep population down is the odd bird/frog/snake<depending where you live and some will get eaten as fry..