Res change last night after lights out. It steadily crept up 250-300 ppm in a weeks time. Part of me wants to assume it's the solubility issue. As it circulates and better dissolves, the ppm go up.
They drank 15 gallons since the last changeout, so they're still taking it in but I think it got too hot.
Last night, I made the same solution but I took a lot more care in the mixing, only dumping a small amount of nutrients into my mixing jug at a time. I mixed, then slowly poured until the sediment at the bottom started to pour out. Then I'd add more water and a little more nutes to the jug and repeat. It took a long time but there was noticeably less sediment at the bottom.
I also used less ph up and down for the intial buffer, thinking I maybe hit them too hard with potassium from the up. It amounted to roughly 100ppm of my last batch. This batch, I used half of that.
Left it last night at 6.1/1090 and it was 6.3/1130 this morning. So it's climbing again. I hope it settles out a lot lower than the 1350 it was when I changed it. Plants are burning a bit but still have great color and vigor, growing inches every day. The math's adding up to them drinking 1.25 gallons apiece, per day.