plankton and tiny organisms are freely found in the ocean. they aren't found freely growing in your tank unless you introduce them. so except for things like clams and sps which rely on zoox for food, you should introduce coral food into your tank regularly to ensure they grow. people put all sorts of things in, phyto, zooplankton, mysis, cyclopeeze, etc. not sure whether all work but probably better than putting nothing in your tank at all.