Sorry to hear that you may have lost your specimen of Sun Coral, I will post my findings all the same if you don't mind as it may help others in the same predicament
My Sun Coral has had youngsters, Not sure how and why (must research)
Yeah, I found that they don't need to be in the dark, for one, how do you feed the thing tucked away under some rock, and being such nice looking things, why would you want to hide it away.
My sun coral is in a pretty heavy flow area, in the open on a rock almost on the sand bed, the flow alternates with the type of controller/pump arrangement I employ.
I found that if you withdraw the flow and re apply it the sun coral will open (providing you have acclimatised it correctly and its settled) If you keep moving it then its not going to settle.
I directly feed my sun coral perhaps twice a week and it gets other foods administered to the aquarium by indirect means (natural feeding)
I feed a range of foodstuffs. Live Phytoplankton, Live Brineshrimp nauplii, Frozen Mysis shrimp, Frozen Krill pacifica, Frozen Brineshrimp and Frozen Cyclopeeze.
It has been attacked by Mushrooms and Rics but has always recovered.
Post a pic Scott all may not be lost.
I feed my sun coral directly with a large syringe with a length of airline attached.
If you have shrimp, no problem, just let them feed from the end of the airline (just depress the plunger enough to let them grab into the tube and collect their own fodder) until they've had their fill. Then you can feed the sun coral without any bother from the shrimp.
If all else fails try Reef Booster to entice it out using the syringe method to squirt short burst over and around it.
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I hope this was of some help to those wanting to keep or are already keeping Sun Corals in their aquaria.
Regards