Sun coral (tubastraea) are non-photosynthetic, so they require
the feeding that the other posters suggested. This makes them different from other LPS
's in which the providing of larger food items such as mysis is largely optional.
For success, feed as many of the polyps mysis shrimp (or other type of food) that you can several times (3x +)a week. Any sun coral polyps not contacted to the main coral body must be fed as well. It's this feeding requirement that makes them challenging to keep, but if they're well-fed they tend to do good.