What is the temp of the tank during mid/late lighting?
If your parameters are what they display up in the start of this post, that is good, but you do not mention the salinity or sg
or Phosphate. Zoos do not like high Phosphate from my experience.
What if any supplements are you feeding, or trying to feed?
The problem with the zoos to me is that the Zooxanthellae is dying off. When this happens, the colony starts to die. Basically starves to death. While there are many things that can cause the symbiotic algae to die off, Detritus, Phosphate, and Temp can be factors to its demise.
Do water changes as others have mentioned, and get them at least mid way up and out of the cyano
.. Nurture them with a turkey baster to get any unwanted algae or detritus off of them... and hope for the best..
I will be hoping with you...