they eat everything you want them to: left over food, detritous, your neighbor's dog......etc.
if you are trying to breed them, get a handful or more adults and put them in a clear container with an aeration stone, and a few plants. feed them a slice of cucumber whenever you feel it's necessary ( i do about once or twice a week). i have over 1,000 in since i started it about 6 weeks ago. (i put 10 adults in there to start) i also put in a bunch of java moss since i heard they like that