I use a medium made of washed coconut husk chips and activated charcoal. That is spread about 1/2-3/4" thick in whatever container I am using. I then keep about 1/4" of water in the bottom and use flake food to feed them. I'm about to start using coos-coos for food because I'm almost through with my flake food that was left over for so long (maybe I'll just use some of my goldfish food since I have so much).
To feed, I just add some RO water to the container, watch the 'critters float to the top, and pour them out into the tank with my frogs--easy as pie.
If you don't start out with a large enough number for your starter cultures, it can take quite a while for them to really take off . . . but once they do, watch out! Maybe someone has come up with a more productive method, but this is quick, simple, and cheap--and you don't have to make new cultures once per month. I can't beat it!
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Homer W. Faucett III, esq.
Purveyor of Trivialities and Fine Nonsense