Plants bottom leaves turning pale yellow/white

[Sistema]

Hey all,
Never grown before and having a couple of issues, hopefully someone can suggest a fix.

So the plant was grown from seed, originally planted it in some seed raising mix where it seem to grow fantastically in the sun.
After it was root bound I transplanted it into a large container with the same seedling raising mix (problem #1 im assuming) and set it under a 90watt LED with an 18/6 light cycle.

For the watering I was using a quite strong (EC 1.8 / CF 18) mixture I use for my outdoor vegetable hydro setup. After a few weeks I noticed the bottom leaves were slowly turning a pale yellow then dark brown patches appearing around the middle of the leaf before turning dry and crispy.

I thought this was from the strong nute mixture I was feeding it so I changed it to PH6 plain water but the problem persisted. But upon further research it doesnt seem to be nute burn as the tips of the leaves seem perfect. Ive been reading through many forums trying to find a similar picture of the problem I have with no luck.
If anything it seems like a deficiency in something.

Also the plant is in a confined space with little to no airflow or fresh air (id manually open up the cupboard for a little bit every couple days), was planning on making some fans to circulate the air when it got larger but perhaps I need to do this sooner than later ?

Ive taken some pictures to see if anyone knows what could be the culprit.

The plant is a little wet as im trying to fend off some spider mites which also decided to make their home on my plant

 

[PeytonManning]

I tired to look at those pics, but they keep showing up really tiny. Can you resize them? Don't want to misdiagnose it. But just from the sound of it, it seems like it could be root rot. Not very many things turn leaves white

 

[PeytonManning]

And how often are you watering?

 

[danknessdude]

Strain info? Have you topped them? If every leaf isn't moving from air flow then you def don't have enough

Also misting a plant is not required. Doesn't need it

 

[Sistema]

Thanks for the replies, if you click on the pics they should come up big?

I've got two buckets inside each other, the inner bucket has holes and the outside one doesn't...
If I pull the inside bucket up I can see roots out the bottom so I guess there's always roots in some of the water which is at the bottom.
I only water it once every week or so... whenever the dirt is dry.

There's absolutely no air movement in the growing area or new air to replenish it unless I open the door, which I do every day. So ill throw a fan in there ASAP.

Not sure of the strain, seeds were sitting around for a year plus out of some random smoke. Haven't topped it yet.
It's only misted up cause I sprayed some Wettable Sulphur to stop some spider mites, never misted them up before.

 

[PeytonManning]

I would get some air moving in there and probably wouldn't let it sit in a reservoir of water. Roots need to breathe air and they'll choke if you leave them sitting in water.