A union blew, don't know how. Unfortunately my return pump was on a battery back up a nd kept pumping water. Ato kept filling it. Approximately 40 gallons went to my neighbor ceiling that I just replaced. Wife said no more, but I can keep my AIO nanos.
Oh, wow! I was just re-reading this whole thread and this was not the ending I was expecting. So sorry that happened. The aquascape and quality of this build was really first rate. I have never seen a union do that either. Just rotten luck.