My experience has been the same as SaltwaterTeen's. There are so many zoanthids, and they come from so many different places, that there are some that like low light and some that like higher light. I've found that generally, the more intense the color of the polyps, the more they can tolerate, or require, brighter light, and a higher placement on the rock. But, when the polyps only partially open, I suspect some kind of irritant, like bristle worms or worse yet, zoanthid-eating nudibranch's...Look closely to make sure you don't have the latter. They're very bad but it's easy to get rid of them. Bristle worms aren't nearly as bad but when they live in a hole in the rock that the zoanthids are on, they're very difficult to get rid of.