Other people have suggested the flame angelfish may be the problem. I've never seen it nipping at the zoanthids, but I don't watch it 24/7 so maybe I'm missing it. Can't see anything else eating them and they looked fine for a while (aside from not being open). They were open when I first got them and have steadily closed over time.
I thought of doing all the tests with Hanna testers but with so many parameters and only 1 test at a time it would take too long (three kids in hockey) so I got the Aqua spin. All the other corals and fish are doing fine and have been for a long time so I don't think there's a serious water issue but will keep an eye on it.