Zoo polyp colonies sudden death

[Acidshock]

I keep getting asked by a friend regarding this matter and I havent been able to find any information regarding this behaviour. My friend has a bunch of zoo colonies and lately they have been dying in groups. For instance he will have a group of say orange polyps and a group of yellow polyps together. He will come back and all the yellow polyps will have closed up and will not open anymore till they are dead. The orange polyps will live. This behaviour has been happening for a while. The weird part is that it seems to be all of the same color within X group of polyps will die. So his zoo colonies are starting to look patchy. I dont see any disease on the polyps; no dots, white spots, etc... Any ideas?

Ph: 8.3
Ammonia & Nitrite: 0
Nitrate: 15-20ppm
Mag: 1400
Calc: 450-490
Alk: 2.7 meq/l (Quickdip reads 180 - 220 ppm)
Phos: 0
Iodine : 0.03ppm

[Wetplants]

First impression is allelopathy. Nitrates a bit high, but zos tend to like that. Does running carbon help?

[Acidshock]

Well my buddy used to run carbon but he switched to a combo of phosban and purigen