DTs is a live cultured Phytoplankton you can buy by the bottle. I personally think its overused in the hobby, as its actually too small for most corals to feed on (much smaller than Marine Snow). Excellent stuff for some bivalves, sponges, tunicates, sea apples, and other specialized filter feeders, but not of any use for soft corals, with the Possible exception of some nonphotosynthetic species.
The only way to tell if a softie is eating generally is to break out the magnifying glass and see if you can see if there is any food moving into the polyp. Some species you can actually see the particle right through the sides of the polyp... cool looking