I fucked up, but I think I caught it in time. Noticed a few dodgy looking leafs last night, so I checked the runoff. It was between 4.5-5 ec on all of them, so I ran about 6 gallons of R/O and 12 tbps of the herculean harvest through them and got the run off down to around 3 ec. Ran my usual 8 gallons through this morning at 1.8 ec and the runoff was at 2.2 ec. So I got it back in range now.
Where I think I fucked up was when the temps started dipping I was only watering once per day, and that was fine. Then when I chucked the spare dehuey in to bump the temp up, I continued to only water once in the morning. Pretty sure I was letting the coco get a bit too dry and it was retaining salts. I'll be going back to twice per day now.