I've had this happen in the past and I later discovered it was caused by a dying birdsnest colony. As soon as I took the birdsnest out of the tank everything stopped dying. I suspect you may have something similar. As soon as you seer a colony start to go, cut your losses and rescue the healthy parts of the colony, if any, and toss the rest. Who knows what caused the die off in the first place.
Alk is high. While it's not catastrophically high, I've had mine this high before and found the colonies to be more susceptible to weird things... In fact I suspect high alk was the cause of my birdsnest collapsing that triggered everything else.
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