Zoas are simple DIRTY water. The problem is so many people want to have a mixed reef. It is much easier said than done because so many corals require different water quality/lighting and flow. Most SPS need the cleaner water while zoas and LPS flourish and thrive in dirtier water, sooner or later you are going to end up pissing something off. I think and this is from all my personal experiences that the easiest way to successfully keep SPS and also LPS and zoas at the same time is to have 2 separate systems. One for The LPS and Zoas that you keep dirtier w/ medium lighting and flow and an SPS that you keep super high flow w/ strong lighting and cleaner water. Having a lot of SPS also sucks a lot of your elements up which other corals need too so unless you are on top of your dosing or running reactors it is much harder to keep those levels in check. I think those are all factors of why zoas melt and don't survive in mixed reef aquariums, especially smaller ones.