leather shreeded

I was away for a few days and came home to find my big leather shredded, one of my smaller ones is gray and my other one looks fine. Nothing else in the tank seem to be having any issues. Actually a while back I lost a ton of zoia's they seem to be coming back on there own. and my anemone has gotten so big he just split his self and both half's are healthy.

below is my water test from today before I did a water change the only thing that jumps out at me is my ALK is low.

PH 8.0
Ammonia 0
Nitrate 0
Nitrite .025
KH 5.7
Cal 360
Mag 1200
Phos 0
Sal 1.024

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