what causes shrinking zoas?

Hello, I have a frag of utter chaos zoas that started off really strong, went from 3p to 11p in a month. then over the last 3 weeks they have started shrinking..they still open but remain super small - see pics below





Any ideas what would cause them to do this? I thought potentially vermetid snails that have grown rampant, there was one in the middle of this frag that I just superglued over to kill but its been a few days and no change.



I have another frag of zoas right next to it that is still open and growing polyps. most other coral seems perfectly healthy - aside from my clove polyps which most are closed except 2-3 are completely wide open..



I run my xr15 about 7 hours a day at 35% - ramping up and down an extra hour or so.



params:

temp - 78

salinity - 1.025

nitrate - 0

pH - 8

phosphates - 0

calcium - 440

alk 6.7



Should I stop doing water changes to get nitrates/phosphates up? I've been feeding what I consider heavy, but doing 15% weekly waterchanges on the tank which is a Nuvo 10, also running carbon - wonder if its just too clean..but I started getting hair algae issues so then again maybe not?

Before and after pics



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njzvKONl.png
 
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