how long have you had your sun coral? i just picked up black sun coral (tubastrea micrantha), and i am waiting for the polyps to open. all i wanted was a small frag, and they sold me the whole piece for 1/2 price.... its 8" in diameter, so i plan to bust it up into small pieces, for better placement in my tank.
sun corals are heterotrophic azooxanthellate critters. they are best kept in a nutrient rich enviroment, and are best fed a meaty substance insuspension. at some point you may need to take the coral out of the tank and place it in a bowl for hyperfeeding, or use the top portion of a 2 liter coke bottle to encompass the coral in the tank, this will help keep the food suspended around it.
hth,
