THC Bomb In Trouble

I'm a bit of new to growing, and I hope some of you more experienced gardeners can help me out. I've done 2 dirt grows before which weren't unsuccessful, but there were certainly problems. I'm now doing my first hydro grow and I've got a sick plant.

The Problem:
A THC Bomb plant is in week 4 of vegging. Some of the large fan leaves, starting at the bottom of the plant, started to yellow between the veins. That yellow is turning to dead brown spots. Pictures attached.

The Setup:
5 plants vegging (18/6 light cycle) under florescent lights (4ft x4, 2 ft x3, big ass cfl x2). Drip irrigation hydroponics. General Hydroponics nutes 'Lucas Formula' (Lucas Formula - Marijuana Growing): Micro 5 ml/gal, Bloom 10 ml/gal, distilled water 3 gal. pH around 5.5-6. Drippers on for 10 min 3 times per day. Plants growning in 4" rockwool cubes inside clay pots filled with Hydroton expanded clay pellets. Temp stays low 70's day and high 60's night.

My Guess:
From what I've found searching the interweb I thing it might be Mg deficiency and the solution seems to be adding a little epsom salt. I'm a little timid to add epsom salt because all 5 plants share the same resevoir and the other 4 are looking fine and I don't want to hurt them by changing anything.

Any thoughts on my situation? Would adding epsom salt hurt the healthy plants? What about misting the leaves with an epsom salt solution? Or am I way off with the Mg deficiency diagnosis?

Thanks for your thoughtful advice to come!

Forgot to attach the pictures...

I've heard people say you don't need cal mag+ with Lucas formula, but I've found that to be bullshit. try 5ml/gallon or use tap water. (We have almost no Ca or Mg in our water out of the tap, so we need it anyway.)

Quote:

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Originally Posted by copobo

I've heard people say you don't need cal mag+ with Lucas formula, but I've found that to be bullshit. try 5ml/gallon or use tap water. (We have almost no Ca or Mg in our water out of the tap, so we need it anyway.)

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Bingo. Cal Mag+ will very likely do the trick.

The Epsom Salts are not a miracle cure thread Too much Mg will cause nutrient lockout. Know what you're doing BEFORE you do it.

Usable light from fluorescent tubes only extends a few inches. Lower leaves will yellow and fade away. Get a bigger light, likely this problem will disappear.

I bought some Cal-Mag+ and added 2.5ml/gal to my water yesterday. I was unsure of concentration, so I started small. The back of the jug gives a desired range of nutrients for a hydro system and then says what 10 ml/gal of cal-mag plus would add. From back of Cal-Mag+ jug:

Desired:
N: 160-250 ppm
Ca: 160-280 ppm
Mg: 45-75 ppm
Fe: 2-7 ppm

10 ml/gal of Cal-Mag:
N: 53 ppm
Ca: 84 ppm
Mg: 31 ppm
Fe: 2.6 ppm

I don't have a EC meter, so I can't check my resevoir ppm levels, but wouldn't that just give me total ppm and not individal element concentrations?

I'd like to calculate what the elemental concentrations from my GH nutes would be, but they are listed as a %, not a mass or concentration. Is 1.5% the same as 15,000 ppm? So, if I add 15 ml of florabloom, and it has 1.5% Mg, to a 3 gal resevoir do i get 19.8 ppm Mg?

(15,000 ppm)(15 ml) = (x ppm)(11,356 ml) [11,356 ml = 3 gal]
x = 19.8 ppm

Is this the correct way to calculate this? Also, if I got an EC meter is there a way to measure individual ion concentrations or is it just total disolved solids? And what good does knowing TDS do if you don't know individual ion concentrations?

Lastly, I'd like to use the Cal-Mag+ to foiliar feed the one plant that is sick. What kind of concentration should I use for that?