In ambient conditions on the reef (with wet bio mass of zooplankton ranging from 0.5-4 mg/L, bacteria 1.5-3 X 106 cells/mL, and DOM ranging from 0.5-2 mg of C/L ), as a rough average - scleractinian corals could compose their energy balance as follows: 60-70% via photosynthesis, 10-20% predation, and 10-20% by feeding on DOM and bacterioplankton (Sorokin 1995).
Clever little fellas are these corals, interesting for all those forum people who say all they feed their corals is light. Indirectly, they are also producing the bacterioplankton and DOC which makes up the balance of their diet.
Does make you wonder though, as long as you are feeding your fish well and providing light, what is the point of all those expensive fancy coral foods?