Zoas can be a bit troubling, sometimes it's hard to figure out why they are mad. When I have a colony closing for no obvious reason, the first thing I do is a peroxide dip using 70% tank water and 30% h202, see if anything comes off of them. If they still don't respond in a couple of days, I will then try a lugols dip. If you think you have zoa pox, treat asap using Furan2. Zoa pox is pretty distinctive, white "pox" will show up on the polyps, and will spread through the colony slowly until it kills the entire colony. Check on zoaid.com for the dosages for treatment with Furan2 for zoapox. If you see white pox, I would treat immediately for zoa pox before it spreads to any other colonies you have. They won't die overnight, but it will kill them if left untreated.
Other things that can irritate zoas - aiptasia (not always), isopods (some people say they don't, but I have seen it with my own eyes), certain snails, zoa spiders (you will get these with the peroxide dip, they can't stand it). Do you have anything in the tank that might be bugging them......fish/crabs? I just evicted my last peppermint shrimp for hassling my polyps, jerk.
And realize, sometimes zoas are just weird and will close up for no reason at all. There is no reasoning with them.
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