In addition to the above, eliminate / prevent pests. Dip them in revive. Look at them with a flashlight after dark.
I never thought I would get zoanthid eating nudibranchs, and attributed my occasional zoa loss to the reef gods. But sure enough, I found one the other day on my glass. If I didn't find him randomly, I would have continued thinking "it won't happen to me".
After I removed the one I went hunting after dark with a flashlight. In about 2 days I've captured about 20 of them, and am about to start a month long treatment of my tank as a result.