FWIW mine is pretty much at the bottom of the tank attached to the base rock. It is somewhat sheltered but still gets a fair bit of light.
I had the same problem as yourself whenI first bought the coral and had it higher up in the tank. No problems since moving it and three heads have split within 4 months. Lighting is same as yours, depth of tank is about 20 inches with the halides about 10 inches over the water line.
HTH,
Rob
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