I have tried algae wafers, bottom feeder sinking pellets, ken's sticks, crab cuisine... they have nibbled if passing, but shown no interest in any of it really. I did add algae powder to the water column, and that definately got them going for a bit.
on a side note, I found the venturi adapter for the duetto and installed it, it's variable flow, so I set it as low as possible to keep noise and bubble chaos to a minimum... seems to have brought them out of hiding a little bit.