I use those little round 200ml pots like the ones you get at the supermarket, weatbix (it seems to work wonders and we always have them in the cupboard), edmonds active yeast and then I throw them in the hot water cupboard and you get a culture in two days max. Smaller containers mean the whole container is covered every day which means the swordtail, cherry barb, platy, guppy, molly and danio fry all get a good feed (there's always heaps of worms!)
I usually change it when it gets manky and there's not so many worms maybe every month or so. I'm lazy and it went super gross and they still managed to survive and make two awesome new cultures (lucky because you can't buy more here!).