First up get some better lighting
Second, why are you turning your powerhead off at night? The ocean never stops moving. I guess you're doing it to avoid getting the nem sucked into it, in which case you should already have it nem proof in the first place.
I'm sorry if I seem like I'm getting up your case but it really annoys me when people buy an anemone knowing full well they can't have one. These are precious creatures and we shouldn't be keeping them in captivity if all we can provide is "good enough". It may be just okay under PC lighting just like we'd be okay living on just rice and water. We'd live, but we won't like it very much.