Full vat. 0ppm tds. Vinegar and tap water bath did the trick. There was a thin layer of nasty on the bottom. I've had this vat for a decade and don't think I've ever cleaned it, but this seems to have worked.
I did finally drain it, scrub it, and run a vinegar/water mix through it. Filling it back up with ro/di now. Hopefully, that knocks it down, and I'll use a trash can I used to use to mix from now on