You can feed them with yeast and algea water or ¨green water¨ as we call it around here.
To obtain said water take a clear container with at least a good part aquarium water.
put in a few drops ofr coffeemilk in it (teh stuff that coms in those litle cups. Put it in a sunny place maybe use an airstone and wait for the magic to happen.
You can also use bloodmeal a single grain of the stuff usually is. sufficient.
we (my brother in law and i) always used aquarium water. However you coul duse part aquariumwater and part tapwater depending on where you are. In large parts of the world that si not a good idea over here it can be done due too the high quality of Dutch tapwater.