Copepods and the other macro fauna in you fuge/display are scavengers and as such will eat whatever they can find. You will be suprised what makes it into the fuge from the display. That said though, if you add anything, live or frozen phytoplankton in small amounts once or twice a week would be the best sollution if one is really needed. Be careful not to overdo it though. The additions can easily lower water quality and increase nuisance algaes.
Cheers
Steve