I have 2 plants on the same system in DWC. Out of those 2 plant one has yellow leafs on the lower half of the plant and spreading up but the tops are looking good. I have flush and redid my nutrients, check my ppm there at 1000.I did that a few days ago. It going on there 4th week in bloom. There other plant looks really healthy. Showing no sign of yellow leafs. Oh ya I have replaced my air stones. .