Anyone know what these dark spots are?

3rd Monkey said:

With Blitz and CannaCountry (how in the hell do you like posts here).

Definitely seems to be ph or your EC has risen and locked it out. Whats your res say?

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oill said:

Thanks for reply. I'm using bcuzz hydro. Rockwool mini chunks is the medium. I usually raise it to 6 as the grow moves to flower.

I now think that the volume of water wasn't enough to get run off. I think it's salt build up. So I have increase the feed time.

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Best of luck, hope it fixes your issue. Watering to runoff is not as important in rockwool as something soil-based. Rockwool has zero cec (cation exchange capacity), so it doesn't 'hang' on to nutrients..there is nothing in the medium to make that attraction happen, like in soil, or even in coco (which has low cec), so a build-up is not an issue like it would be in soil.