Only the water moved. Nothing else.
Assuming the DT doesn't start going south again:
Things that are NOT causing the problem:
In DT:
water source, return pump, tank, silicone, vortechs, lights, ATO, sand, rocks, fish, bugs, cyano (if anything when the cyano showed up, things started looking better)
in QT:
water source, heater, AC50, MJ400, lights.
Things that MIGHT be the cause (all offline now):
eshopps skimmer, hydor inline heater, tubing that connects the return pump with the heater and the pvc for the return, mj1200 on reactor GFO
At this point, my best guess is something was leaching enough tiny amounts of bad stuff in the water that caused the problem. By moving that to the QT, the problem moved as well. By doing the 50% water change on the DT, it was diluted enough to reverse the process.
My current plan is to not do anything for the next 3 weeks (partially because I will not be here

). If the monti in my DT keeps reviving as it does now, I'll put back the LEDs. If it goes to bad state again I'll still put back my LEDs and continue with the equipment changes.