Depends on the type. Hairy types like the Ricordia's do take small (very small) samples of brine or mysis. The flatter types, Discosoma variety, with no visible polyps are mostly photosynthetic, and absorb what they can filter thru the water. You do not have to directly feed any of them. Mine multiply rapidly, but I have a very mature tank. Depending on the type, and age of the tank you may have to feed filter type foods.