Makes a lot of sense, these babies went from organic nutes in dwc (I had tons of issues as I was warned I would have), I flushed for about 4 days and switched to GH Flora Micro & Bloom using the Lucas Formula. Everything improved and I vegged for a few more weeks and then flipped it.
I have completely drained the res (Once the initial problems were worked out) about 3 times. I have always done the addback w/tap and my PH has always been ~5.5-6.5, until they started sucking down water. I have just been doing some quick reading and I agree with your suggestion to run balanced water once a week.
Do you think at this point I should add nutes back to the res, or do you think I should just run plain ph'd water until harvest? I have to harvest in ~2 weeks no matter what.