HELP! Leaves getting brown grwoing spots

[cptgold]

Hi,

Would really appreciate some advice on how to stop the cause of these brown spots.

I'm on my second growth now, after upgrading the light I'm stunned by the growth of these girls.
But yesterday night I noticed brown spots on two fan leaves on my White Widow Automatic, pictured in the closeups. The picture of the full plant is from today.
I moved my two plants, the other one is a Royal Jack Automatic btw, to bigger pots 3 days before I noticed the brown spots.
Using the soil from same bag as they were previously in, however, this one got about 15% old dry soil from another bag closest to the leca's in the bottom, as I noticed I didn't have enough. I watered it a bit first to rehydrate it.

Plant
White Widow Automatic Seed
3 weeks old
10 cm (4") tall

Soil
Ecologic soil for herbal plants, enriched with chicken manure, peat, clay, lime, silicon, iron, magnesium, calcium.
Leca in bottom for drainage and oxygen, plus a handful spread into the soil.
10 Liter pots (2.2 gallons)

Light 24/7 (Both plants are Autos)
1x Mars-Hydro TS 1000 LED 150W Compares to: HPS/MH/HID:250W
60 cm (24") hanging distance from top of plant.
Two white walls, two of natural wood.

Ventilation
1x 107 m³/h exhaust fan, grow room is 0.63 m³
1x Oscilating table fan, at intake, giving the plants a fresh breeze

Climate
25-27 °C (77-80 °F) 35-45 RH% (I'm struggling to increase the humidity)

Water
Tap water, no added nutrients, no flushing, let top 3-4 cm (1-2") of the soil dry before watering

I'm planning to improve their environment with a growing tent, for better light reflection and easier climate control. But pretty limited by insane shipping fees to Sweden and a small 61x61 cm (24") closet.

 

[SmashGuitar]

IMO i can see some water on your leaves. With a bright LED light it magnifies the light and can burn the leaves.

 

[Daskahn]

I was going to say the same thing as smash..
don’t let any nutritional water get on the leaves spray with regular water.. and as the lights are about to go out.. never while they are going to be on.
. to fix your shipping problem..

go down to your hardware store get piping of your choice and make a cube.. then go down to your camping store and pick up Mylar blankets.. attach reflective sides toward the inside of cube.. hang light bam you got cheap diy grow tent.. won’t be as good but better then you would think

 

[cptgold]

Sorry guys, really thought I had posted a reply already.

IMO i can see some water on your leaves. With a bright LED light it magnifies the light and can burn the leaves.

Made sense to me, so I've made sure not to splash any water on the plants the past two weeks. It has still been growing pretty well, even compared to her very healthy cousin in the back (not same strain). However some leaves keeps going brown, after a few days they dry out completely until they crumble as you touch them.
It really looks like rust on the leaves, but no iron taste to the water, and the other plant gets the same water.

I was going to say the same thing as smash..
don’t let any nutritional water get on the leaves spray with regular water.. and as the lights are about to go out.. never while they are going to be on.
. to fix your shipping problem..

go down to your hardware store get piping of your choice and make a cube.. then go down to your camping store and pick up Mylar blankets.. attach reflective sides toward the inside of cube.. hang light bam you got cheap diy grow tent.. won’t be as good but better then you would think

They're on 24/7 light, but I could turn it off to spray them. But I haven't sprayed them at all past two weeks.
My first plan was actually to build it myself, but the price of the materials in Sweden is about three times more than what I finally paid for a tent from German Amazon, incl shipping.
This is two weeks after OP, just moved in to the tent. Both plants are of the same age, so not much wrong with it's growing speed, but the brown leaves keeps appearing

 

[Daskahn]

Salt build up?

 

[SoOrbudgal]

Do you have holes in those plastic pots it looks like the roots have cooked and the plant has root rot. This looks like overwatered and lack of drainage.