First thing I would do is elevate them off the sand which can irritate them. Lower third of tank best. Water quality also important. Good test kits are a must to monitor tank. . .. Fish dont talk - Test kits DO and stay away from API kits.
I would highly suggest taking a water sample to a trusted LFS that does NOT use API kits and see what numbers/readings they come up with and to compare with yours if youre using API.
There are a number of factors why zoas close up. Some are water movement/flow as zoas do not require the consistent high flow conditions that SPS corals do. I would consider a moderate flow environment ideal but Zoanthids, like most corals, can adapt to low or high flow. In high flow, you will typically see polyps grow closer to the rock with shorter stalks. Another is lack of feeding and food as infrequent feeding and ultra low nutrient conditions can lead to entire colony meltdowns. Target feeding is not a requirement as Zoanthids are photosynthetic. I have found that target feeding Zoanthids always provides mixed results, when a food particle falls onto the polyps.
Back to parameters, good water quality is a must.
Alk: 8.0 - 11.0
Calcium: 400 - 440
Magnesium: 1300 - 1350
Iodide: Maintained via regular water changes or manually at small dosages
Temperature: 77-79 degrees
pH: 8.1-8.3
Phosphates: .04 - .06
Nitrates < 10
Asterina stars, little tiny tiny spiders and nudibranchs also will make them miserable to point of death as will aptasia, worms like spinoids or vermetid snails.
