Where is your sun coral located in the tank? It needs to be shaded...they don't like bright light. If it's in a brightly lit area, that may be why it's not eating. Mine is shaded pretty well and it usually only comes out after the lights have gone out. How long have you had the coral? It may take a week or two for it to get happy and decide to come out. Mine stayed closed up for about a week at first. I feed it Kent Phytoplex, CS Live Coral Food, newly hatched brine shrimp, and my custom mixture which consists of whatever I can find in the seafood section at Krogers. I dump it (shrimp, clams, scallops, fish filets, OSI marine flake, Seachem Reef Plus, ect...) into the blender and puree it. I also mix some clam juice with it when I can get it. Then I freeze it in ziploc bags for later use. I just break off a chunk, thaw it in some tank water, and dump it in. You have to be careful with this as too much can foul the tank. If the polyps aren't at least partially extended, it won't eat anything. You can try to entice it with a small amount of food and see if it will open up some...that works with mine sometimes when I can't be here at night to feed it. Good luck with it and keep us posted

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Logan J