Rusting leaf veins

[toblue]

A few of the mature leaves on my plants have rusting leaf veins. The plants are due to be chopped and will be flushing in a few days.

I don't think it is a big problem as it is only a few leaves and the plants are otherwise very healthy.

I did at first think a classic calcium deficiency but am a little baffled as to why it is only running along the veins. I do live in a soft water area and ran out of calmag a few weeks ago.

Strain is Black Dog, Coco, Feeding at 1.1 to 1.2 ec 5.8 to 6.2ph. I know it is to late to do anything about it but would be nice to know what it is.