Zoas are hardy yet can be tricky but with a 3 year tank - should be overly mature supporting most anything.
First to say, buying quality test kits are not just a must but should be a priority as it seems things go wrong on a Sunday night when all stores are closed and the first thing we do when issues arise is to test water. If it API kits you are using- there is a high likelihood you ARE GETTING FALSE READINGS. So lets skip the speech on water and talk about zoas. Zoas must have iodide, not iodine in their water and under moderate light and water flow. They seem to do best in the lower half to third of the tank in an area of low-medium to medium-high flow. Too much flow may make it hard for the polyps to open. You will know your zoanthids are ‘happy’ if they open up and are fully extended without seeming to stretch so far upright.. The majority of zoanthids have symbiotic photosynthetic zooxanthellae (I dare you to try and say that three times quickly) and are therefore best kept with at least moderate aquarium lighting as mentioned for production of zooxanthele.
Zoanthids require the standard water parameters that are good for keeping just about any of your corals healthy. You’re aiming for water temperature about 78 F, salinity somewhere around 1.025 specific gravity) and normal hardness 8-11 dKH) and a pH around 8.1-8.3.
However, if your polyps were previously open and have recently closed up, this should be taken as a serious sign of a water parameters issue. The first things I would check are ph and salinity. In my experience, zoas will close up if there are swings in salinity.
One disease that seems to disproportionately impacts these corals is something called zoa pox or zoanthid pox. Zoa pox is the name given to the zoanthid disease characterized by tiny growths on the side of the affected zoas. I’m not sure whether the growths/pustules themselves irritate the polyps and cause them to close up, or if the coral is otherwise sickly and closed up (therefore showing the zoa pox), but the bottom line is that if you see zoa pox, you have a sick coral.
Lastly, check for Super tiny spiders or nudibranchs which will take them down in No time. If you see these pests, start looking for eggs which are future pests to cause further problems. Hope this Helps !!!