In week three of veg, have 4 plants in a 8ft X ft8 tent, all showing this signs on a few leaves. This has happening over past 36 hours since the last feed, they were fine before that.

In Coco pebble mix. Feeding at around 2.4ec as they're using co2. Have been adding a little extra calmag in addition to the a+b. Ph was around 5.8 going in, but that may have been wrong as I'm not confident oh pen was reading accurately, it could've been much lower, like 5.2. 300w led lights, currently set at 60% and having about 36" above canopy.