Hello Sarno, sorry to hear about your zoa colonies dying. How closely have you examined for pests? Have you added any new
frags to your system that could have introduced
nudis, spider or zoa eating amphipod/ do you dip your newly aquired
frags or give a
FW dip with
RO/DI water? Has anything changed in your routine that could cause you to take a closer look at? Can't answer any lighting Qs as my knowledge is minimal at best. Your overall tank "looks" healthy and hopefully it's a simple solution of lighting, flow, or water imbalance. (Are all your water parameter readings good across the board with the exception of lower than normal
alk? How about the calcium?) Zoas have been known to get the "pox" and also a "fungal" infection. Some species are more prone to these than others. Sometimes temp swings will cause them to become sick. It has been said recently by some zoa hobbiest that the reference "beginner coral" is not correct, as some species have been an ongoing trial to keep.
I have read some zoa hobbiest will start fragging off of colonies that are having problems in hopes of saving some specimens in a "worse case scenario". Not saying that this is something you should do by no means. I'll leave that up to the "Zoa Gurus" to advise you.
You just mentioned that you had some greenbacks tied up in them. Just sharing some material I read with you so you can do further research on your own. And anything you can share would be welcome also.
Just Something For The Zoa Data Base 
I ran across a bazaar thread in my surfing through zoa hobbiest and there was a thread on a dying colony which was pulled out of the
DT for a closer inspection to find that the rock smelled bad, like rot. He had to
frag the whole colony in an effort to save it. It never said what caused the deterioration of the
LR. I don't know if it was caused by a baterial infection of the
LR or exactly what the culprit was. Just found it interesting as it was very unusual.
