[Elliott]

Well, left town for a week, came home and found my sump tanks almost dry, no return to the display. Display's only circulation was the closed loop. and since the chiller is fed from the sump system, which was now isolated from the display, the display warmed to 85. So far, only loss is an acro colony, bleached white.

Culprit?
I inadvertently bumped the switch on the power strip supplying the I0/DI unit prior to leaving town. So, the system went without top off for 8 days. Evaporation ensued until the pump lost it's prime, resulting in the sump loop being isolated from the display.
Remedy
Will not use power strips with on/off switches

I had one of my employees watching over the tank, provided with a detailed list of items to look for, but the problem didn't manifest itself until the last 24 hrs of my trip, when apparently the water level in the sump dropped to the point where it could no longer feed the display.

I'm lucky I returned when I did.