Plz Help I Dont Have a Clue Pics +Rep

Hi all,
Right first things first they are 5 and a half weeks old been on 12/12 for 11 days and have just started to flower.
The problem started in week 3 when most of the big shade leaves started to yellow and then just die off.

Im sure its not nute burn i ran them on just water for a week and a half with no change.

Temp- 30'
Humidity 35-55
Ph- 5.7-6.9
EC- 18-20 using canna Products

I think its i deficiency of some kind but this is my first grow so im a bit clueless

Please Help Me

+rep for anyone that posts

where did the yellowing start? did it start on the tips of the leaves and progressively get worse? or did it just appear that the whole leaves got yellow over nite??

Magnesium (Mg)
Magnesium is a component of the chlorophyll molecule and serves as a cofactor in most enzymes.
Magnesium (Mg) deficiency:
Magnesium deficiency will exhibit a yellowing (which may turn brown) and interveinal chlorosis beginning in the older leaves. The older leaves will be the first to develop interveinal chlorosis. Starting at leaf margin or tip and progressing inward between the veins. Notice how the veins remain somewhat green though as can be seen in figure 15.
Notice how in figure 16 and 17 the leaves curl upwards like they're praying? They're praying for Mg! The tips may also twist.
This can be quickly resolved by watering with 1 tablespoon Epsom salts/gallon of water. Until you can correct nutrient lockout, try foliar feeding. That way the plants get all the nitrogen and Mg they need. The plants can be foliar feed at ½ teaspoon/quart of Epsom salts (first powdered and dissolved in some hot water). When mixing up soil, use 2 teaspoon dolomite lime per gallon of soil.
If the starting water is above 200 ppm, that is pretty hard water, that will lock out mg with all of the calcium in the water. Either add a 1/4 teaspoon per gallon of epsom salts or lime (both will effectively reduce the lockout or invest into a reverse osmosis water filter.
Mg can get locked-up by too much Ca, Cl or ammonium nitrogen. Don't overdo Mg or you'll lock up other nutrients.
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this is what it looks like to me

That is either a mag def or a potass def. check here https://www.rollitup.org/marijuana-plant-problems/157345-have-plant-problem-check-here.html

The yellowing starts between the vains and then the edges burn and die they where fine till week 3 here is a ic of week 3

i'd say robert nailed it... looks like mag def to me.

what stuff can u use to take care of a mag def. without bombing the plant with stuff it doesnt need?