I'm really a newbie at this, but I just went through the same process you're describing, and had the same question. Based on the advice of others on this forum I haven't put a drop of food in the tank..well, except for some DT Photoplankton for a clam that we discovered. The "critters" - several snails, a couple of little hermit crabs and a peppermint shrimp, have been eating algae, junk on the LR
, sand, etc.
At the end of your cycle you should see some algae and diatom growth (brown stuff), I guess that's pretty normal. The critters will eat up that like crazy.
If I'm saying the wrong thing I'm sure you'll get excellent advice from others.