I'm currently dealing with something very similar. Currently 5 weeks in to a Cheese XXL grow by Dinafem. I'm using HLG Quantum Board LEDs, Pro-mix HP, GH Flora Series nutrients, and 5 gallon fabric pots. RH ~45%, Temp ~ 25c.
The two contending causes I'm considering right now are a calcium toxicity and/or a phosphorous deficiency due to a pH that is too high. My tap water is around 30ppm and has a pH of 8.0. I've been adjusting to around 6.5 with apple cider vinegar, but I'm wondering if I should be using something more suitable to growing cannabis. Today I picked up some RootFarm pH down, which is primarily phosphoric acid. I gave my plants a flush with water in the range of 5.6 - 5.8. I went lower on the scale to potentially counteract what I had believed was a too high pH, and I'm seeing a run-off of 6.4 and a medium pH of around 6.9, for what it's worth. I'll report back on how this goes.
As for my speculation of excess calcium, I was originally under the impression that Pro-Mix was HP was essentially peat moss and perlite, but I've since found out there's a week's worth of starter nutrients in there, along with dolomite lime. When I planted this seed originally, I was unaware of this and started 1/4 strength nutrients immediately, with fresh water in between BUT I did add half-strength Cal-Mag with every watering. I'm wondering if Cal-Mag with every watering + the dolomite lime that was already in there might be the culprit. I've seen pictures of calcium toxicity elsewhere, and it certainly does look like what you and I are experiencing.
-tronN00dles
www.autoflower.org
