Sickest plant ever? I need help plz..

[Biic]

hello, ahhhh. so this is my 1st grow and it has been a disaster to say the least. I currently have 2 Northern Light Auto's going. the bigger one is on day 42. little one is on day 18. The bigger one/the 1st plant has had its fair share of problems and was stunted due to over topping &experimental dumbness. I noticed yesterday she was really droopy and the fan leaf stems started turning purple. i also noticed she was REALLY dry but was going to wake up this morning and give her a feeding. When i woke up i noticed ALOOOOT of yellow leaves and she was looking/smelling like a dried up stem. I got in shock and ultimately thought it was due to under watering so i gave her a gallon of water PH'D to 6.20 with 5ml tiger bloom and 15ml big bloom. i just recently stopped using grow big and switched to tiger bloom last week. the day i took her off veg nutes i started to suspect she was acting different. then started the yellow leaves slowly setting in. mind you, the 2nd/little plant of the bunch looks great compared to what the bigger one looked at that age it is about 3 inches taller and the nods look more spaced, no problems to me at all so far and has been on the right path, i think? anyways, if anyone could help me with this issue and what i should do next i would greatly appreciate it. again, this is my first grow so this stuffs bound to happen. should i remove the yellow leafs? is she gonna make it? was is a under watering mistake or all of the above? im a noob and i will greatly accept any criticism thrown at me. thanks again for all the help! below i will leave some information about the grow/plant and a couple pictures (the last picture is the smaller plant i think is doing alright).

-every feeding has been a PH of 5.7-6.4
-light is 18 inches above big plant
-light is a QB288 V2 RSPEC ran at 167watts (was previously ran at 120watts up until a week ago)
-grow box is a 2x4x5feet tall box with white paint
-only one small outtake fan (getting a inline with carbon filter next week)
-temp has been steady between 77F-81F with a humidity of 30%-55%
-light is 20hrs on and 4 off since germination
-Fox Farm Happy frog soil with 3 gallon fabric pots since germination
-fox farm trio nutes. used grow big and big bloom first 5 weeks then switched to big bloom and tiger bloom last week. very very low ML's of both though.
-feed them every 1-5 days 1/2 to 2/3 a gallon of water. mostly 1/2 gal of water every feeding. but big one took 1 gal water today and drained 10% to bottom pan then sucked it right back up about a hour later??? (this is what made me believe its from under watering)

any more questions feel free to ask! i tried leaving as much info as i could. thanks again everyone who see's this!!

 

[Kyote]

I'm on my first indoor grow and growing NL also. I have made a ton of mistakes also, from lighting distance to watering to nutes. The one item I had left out is Cal/Mag. From what I have read, plants need cal/mag that are growing under LEDS.

I was using the same nutes as you that I had left over from outdoors. Just a couple of weeks ago, after more reading, I switched to the Greenleaf Nutrient lineup incuding cal/mag pro, Bud Explosion, Sweet Candy, and the base which is Mega Crop. They will make it to the finish line now. If you didn't cut the Fox Farm soil with 40 to 50% perlite, my hunch is the soil is retaining a lot of water. I know I had to cut it when using outdoors. You may want to invest in an inexpensive moisture gauge for the soil. About 9 bucks or so on amazon or ebay. Takes the guesswork out of watering. Don't take my words as any kind of knowledgeable person when it comes to indoor growing. Learning here everyday. Good growing to ya!

 

[Waira]

-- looks like she's needing more nutes... How many ml's exactly are you talking about here? HF isn't very rich soil,...

 

[Asgardian_Grower]

Lack of Air to the roots, phosphorous looks a bit locked and maybe nitrogen - if your roots have been taking up what leech is always there - could be sensitive to salts - the young one isn't experiencing anything yet - if the soil was too hot - it would have burned - so - I would say flush her with 2 to 4 times the water volume capacity of the pot your using - but - pop your pot down and take a look at the roots - look for browning and smell the roots. should smell like alfalfa - root issues can mimic deficiencies and toxicities - do this before flush. when popped - also look for salts along side wall of pot and crystal growth on substrate that touches the roots.

1. Look at roots
2. look for salts on pot
3. Get Air Pruning Pots
4. Drench - if necessary
5. let me know how it goes in a week