Brown tips?

[Rye]

Could someone offer some advice into what caused this? I'm using a DWR with chard, romaine, and seberrian kale on the table. Has been as low as 660 and up to 170x10 ppm based on my pen. I read lettuce likes 660 ppm but it seems to respond better at 120-140x10 ppm.

The browning did stop when I changed the two week old water in the tanks. I am looking for a more solid answer than " bad water". I'm trying to document everything so it is repeatable if desirable and a list of don't do's.