They'll be feeding on 60-100micron zooplankton, which is bloody small I bred redfin bullies, and couldn't bring any of the fry to adult hood due to lack of food.
I tried using green water, which had all sorts in it, micro algae's, tiny zoo plankton but to no success. Commonly the fry will travel out to sea for a short period of time, feeding, before then returning back into the tributaries. However, Crans and Commans are known to reproduce, successfully in ponds, so do not necessarily need to feed on whatever it is they're feeding on at sea.
Try collecting pond water, culturing it in a larger container and placing eggs in that, probably your best bet!