I'm with you - confusedOk so, me being me, I also asked the soil company their thoughts. Here is what they wrote:
"From looking at this picture it almost looks like overwatering. By how the plants look burned on the tips and are browning like the soil is burning them. Also check your lights, another thing that could cause this leaf damage is lights too close or too bright. The leaves also look drooped, which could mean over OR under watering. Most every issue with these mixes comes back to watering.
Before adding anything try decreasing the amount of water they are getting. Also the days in between watering could be too few. I have seen this before with pots that where never allowed to dry out sufficiently. I would back off watering until they are bone dry.
The odds of the plant being deficient if its in the soil are slim. Unless the pot size is too small for the plant/environment, and that plant has been in that pot for far too long. What size pots are these currently in? Also what size pots are you vegging in?
The only thing we would ever recommend adding is Compost or Worm Castings as a top dress or foliar feed. Any Teas are very beneficial. Or any microbes are very beneficial to the soil. I doubt adding Cal/Mag or any other nutrients would do anything but further burn these plants.
For the most part you caught the problem soon which is good. Those plants look good, we will diagnose the problem and you will finish strong!"
Hmmm, now I'm really confused, any thoughts @StoneOtter @Emilya ?
Not terribly sure it's overwatering. They get the same water, 1 gallon every 2-3 days depending on how the soil feels. We test the weight of the pot, stick fingers in it above and below to determine. Usually they are pretty damned dry when we water.
That's nothing like over watering in my book
Looks more like pH drift or mobile nute hungry - CaMg?