This is taken from Saltwaterfish.com.
"These copepods eat detritus, left over fishfood, fish waste, plant debris and microalgae." I also found that they will eat "phytoplankton" from livecopepods.com.
They can eat phytoplankton but not nano. Nano has a very hard cell structure compared to other phytoplankton's. Its pretty much only good for feeding rotifers.