Hey dude I built and ran rdwc for a while, and in my experience, ph drop meant either food strength was way too low, or you have bad stuff happening to your roots. RollinEndough is absolutely right about undiluted stuff added to res frying roots. Once you have damaged roots, pathogens have the opportunity to attack. Then ph drops like you are describing.
I moved away from rdwc because it is so touchy. I now topdrip into a redi-roots aeration frame suspended in the lid of a 27 gal tote. There is an inch and a half of water at the bottom of the tote. I aerate this water. Topdrip reliability, and once the dangling roots hit the water, they fucking explode. Since each station is a standalone unit, non-recirc, no pathogens get spread to everybody else. My shit survived 77 degree irrigation temps with aplomb, and the 1.5" of water helped them survive a 90+ degree day in the room, unscheduled, due to a mishap.
You can find info(pics) in my grow diaries somewhere, super cheap and easy to build.
It is the highest-performing 100% reliable system that I know of. It was built so things could go wrong, several things could go wrong, and I would still get a crop out. It works like a fucking charm.
Rdwc is cool too, but holy shit is it easy to make plants stall or take a shit with that system unless ALL your ducks are in a row.
I can move my plants. Flexible irrigation and drain tubing. Can't do that w/rdwc.
All of us who ran rdwc have wondered, as we pulled up to the site, what we were going to come to. "Fuck, I hope they are all still okay." That thing. I don't do that anymore.
My system beats rdwc on an annual basis because unlike rdwc, you have to try like a motherfucker to make plants stall in it. You have seen stalled plants, right? When you are trying to get a clock ticking on a paycheck?
Oh and I can use teas and organics with absolutely no fear. Can't do so much of that w/rdwc.
Pretty fucking happy with myself right about now. I fucking love being right.
Hope some of this helps.