What kind of lights do you have it under?
Most of the time they do just fine without feeding, but they will grow faster if food is offered on a regular basis. Marine snow, cyclopeeze, small mysis shrimp- all work reasonably well. DT's is a brand of live phytoplankton; it is far too small to be used by frogspawn, or most any other stony coral. It is best used for some filter feeders and to give your copeopod population a boost.