Sun corals can be trained to be open while the lights are on - IF you are going to feed the tank at the same time each day - by first 'teasing' the coral to open by putting a tiny bit of food into the water - use Zooplankton or a few pellets or pieces of flake food.
Once the coral realizes there is food, it will tend to open up so it can eat. But said I said, the trick is to do it at the same time each day and eventually it will automatically open at that time. It may take a few weeks.
Many folks have an issue feeding these as other critters/fish/shrimp will happily swipe the food the coral has captured in its tentacles, so it is suggested that at least a few times a week you make sure the coral is eating a good meal.
You can do this by cutting the 'cap' portion off a 2 litre soda bottle (or a 16oz bottle if you have a smaller coral), by slicing it about 4 or 5 inches from the top so you end up with a dome with the opening (where the cap was). Find yourself a
glue gun and hot glue a few marbles or pieces of rock rubble near the opening to weigh down the dome. Place it over the coral and squirt the food in there...a turkey baster is handy for this.
IF you have no critters in the tank that will steal the food, you can simply direct feed the coral with a
gentle squirt of food from the baster.
You do want to make sure all of the heads get fed.
One other thing - sun corals do NOT need to be kept in the shade, nor do they need to hang upside down at the entrance to a cave or pass-thru. They commonly do this in the wild as the currents tend to flow through these quickly and bring small particles that the coral will eat.
They can very happily live out on your sand bed (or in your aquascaping though it is harder to make sure they get fed if they are in the rock work)
Hope that helps!
