zoas receding

[reefndude]

Recently I've had some of zoas close up and few have receded. Only the ones that have been in tank for at least 6 months have done rhat. Have not seen any signs of pest or anything.
Been going on for a few weeks. Any ideas what could be causing this? All parameters are normal

 

[Reefing Madness]

 

[1mbrews8]

Post some pictures and tell us what your parameters are exactly please.

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