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A ph issue is possible, using that garbage soil will cost you because we are not able to predict it's content or reaction based on experinece.
Personnaly adding bone or blood meal, kelp meal, EWcastings, dolomite lime @ 2tbs/gal of mix is the backbone of a proper organic routine,i use double coverage ie:having a source of N-P-K + micros/macros mixed in the soil and a liquid form of these to feeed once the solid soil sources can't keep up or are exhausted.
Superthrive will not supply N-P-K the meat and potatoes of what a plant needs, FF Grow however should but I've never used these.
Unless your water quality is very poor I don't think it's the water, i say the soil is too inert and poor in humus and barky (is that a word...).
To test ph of that soil put some in a container with an equal volume of RO water and shake for a minute and let it settle, then measure the ph of the water that seperates from the soil, it's a good start to knowing what's up.
If all else fails i would transplant them to smaller containers with a top quality soil.
I keep my MH at 36", besides that won't cause yellowing or lockout but rather it will stretch.
thats really hott dude seriously with mine im looking 20 degrees under 250 and dude that is unusual i think try cooling them down off the neuts for a while let the leaves get a lil greener the greener the leaf the more light it can take and i have found sometimes conditions can do that try take one out for a day leave it in a really mild breeze maybe beside and open window be careful yeah and give it a drink and leave it for a day i know it is not ideal man but if there goin bad give it a try it definatly wont kill them just a suggest let mme know if u try
your seedlings are terrible for there age.
1st thing your potting mix you should know the ph and nutrient content. Your water you should know its ph and amount of carbonates sulphates ect .
next light intensity and spectrum,use a metal halide for veg.
Important root to shoot ratio ,to much light with seedlings will lessen the amount of root growth which will retard shoot growth.
If you had chosen a good quality seedling/potting compost you would have no need to feed atall for 2 weeks at least you could have just ph'd you water.
And what is the air like in your grow area.control the humidity around the seedlings they like being misted.
my seedlings are usually 12 inchs tall with a stem diameter of 3/8 or more at 21 days and at least half of them would have shown there sex
Maybe your money would have been better spent on some wood and rope and a couple of pulleys you could of built your self an arse kicking machine!