The problem with sun corals is that when we get them they are already starving and the process to get them to open in very long one around 1 month and it take up 30 mins to 1 hour... You will need to remove them everyday and put them into a bucket where you will dump food in and wait for them to open and feed them and this must be done in the same time so that they will get used to the timing... This is one way if your sun coral is big.. If it is small, place it in a open space in your tank, place a coke bottle that is cut into half and cover the sun coral with it and pump in the mysis into the enclosed sun coral.. Best do it when the light in dimmer or at night, This will encourage them to open readily.. Slowly they will start to open when they "smell" the mysis and slowly, they will start to open daily.. Success rate varies, it all depend on the specimen that you got from the LFS.. Get one that is still full of meat at the coralites and not receded without any flesh..