i'd use (and have used in my minnow spawning/marmokreb breeding tanks) sponge filters... safe for fry, but you'll end up having to feed the tank your green water more often... maintenance is squeezing out the filter every couple/few weeks in some clean water - non chlorinated
one thing i did notice with my rosy red fry was how there would always be several tiny little minnow hanging around the snails (that i got from duckweed i bought).. i know i read something somewhere about the snail waste, and now i'm thinking it was something i read about gamarus (scuds).. anyways, yeah.. sponge filter, just hook up to an air line you can regulate
