It could be a number of things that can be affecting your zoanthids...for one you may need to check not only your pH but your nitrates, nitrites, salinity and ammonia levels.
I remember when I started I noticed my zoo's showed the same symptoms, then I started to test my water and not only was pH down but I had traces of nitrates, nitrates and ammonia in my tank which resulted in my zoo stock dying. Get some Salifert test kits and perform water changes, if you can't do that then I would suggest getting some of your water to your local LFS to test.
