YES! It all makes sense!! I went back and checked my records... My first large tank (125g) was a resounding success for the first year or so. I had coralline to no end, I had every coral from Zoas to thriving Acros, it was great. And then, about a year ago, I began my ATO days and no longer used buckets and making 5g of RO/DI at a time. I used a Brute to store said water... Never again did I go without on a system (now they're even my sump!!), and every system since has been fraught with phosphate issues and coral problems. I have eliminated everything BUT that because it's so inconspicuous... I mean, it's the ATO, right?
Well, this all ends tomorrow. I'm picking up a 75g glass tank for my new sump at the LFS, and I'll just use a spare 20g tank or something to hold RO/DI water for top-off and do so as needed until I get a nice glass tank to fit up on my water tower.
The documentation of this transition begins.