LEaves curling up

I've been having a situation with 1 of 2 plants currently in my micro grow box. Both are the same age and strain.
The problem as can be seen in the picture is a curling of some leaves, It's happened 2 weeks before and it went away as soon as I transplanted from a 16 oz cup to a 2 liter pot, 10 days later it happened again, and I'm not sure if it was the transplant that helped or the watering that came with it.


Currently growing in SOIL:


How long has this problem been going on? It started 1 day ago, although it happened 2 weeks ago and lasted 3 days until I transplanted from 16 oz cup to 2 liter pots.
What STRAIN are you growing? World of Seeds Colombian Gold Feminized.
What was the establishing technique? (seed or clone?) Seed.
What is the age of your plants? 28 days old since breaking soil.
How long have they been in the soil mixture they are in now? Since the beginning.
Were they in the same mixture when they were seedlings/smaller plant? If not, what mixture were they in before? Yes same mixture.
How Tall are the plants? 8 cm, just over 3 inches.
What PHASE (seedling, vegetative or flower) are the plants in? Vegetative stage, dropped the cotyledons 4 days ago.
What Technique are you using? (SOG, SCROG etc) I only have topped once around day 16, topped the 5th pair of leaves as it was appearing.
What size pots are you using? (Include how many subjects to pot) I started in 2 oz. pots, 9 days after breaking soil transplanted to 16 oz cup and 9 days later I transplanted again to their current 2 liter pots.
What substrate/medium are you using? What brand of soil mixture are you using?(percentage of perlite, vermiculite...etc?) Biobizz Allmix.
What brand Nutrient's are you using? So far none, just plain water.
What is the TDS/EC/PPM of your nutrients used? I’m using tap water which is naturally soft, 20 ppm (or 0,04 EC).
What is the pH of the "RUN-OFF"? Haven’t measure pH, mainly because the water is very good here.
How often are you testing pH/ppm/EC/TDS? pH, never. EC every time I water.
How often are you watering? I water every 3 to 8 days depending on how most I notice the soil.
What size bulb are you using? I’m using 6* 10 watt led bulbs.
How old is your bulbs? 1 month old.
What is the distance to the canopy? 7 inches.
What is your RH Factor? (Relative Humidity) Between 40% and 60%.
What is the canopy temperature? 23C to 26C or 73.4F to 79F during lights on. During lights off it stays between 17C to 19C or 62.6F and 66.2F
What is the current Air Flow? (cfm etc.) 17 CFM
Tell us about your ventilation, intake exhaust and when its running and not running ? Passive intake with exhaust always on.
Is the fan blowing directly at plants? No fan blowing at the plants.
Is the grow substrate constantly wet or moist? No.
Is your water HARD or SOFT? soft
What water are you using? Reverse Osmosis (RO)? Tap? Bottled? Well water? Distilled? Mineral Water? Tap water.
If using tap water, what is the ppm/EC/TDS of the water right out of the tap?(Only if you have a tds pen) 0.04 EC or 20 ppm
If using RO,Distilled,mineral water, what is the ppm/EC/TDS without any additives?(Only if you have a tds pen)
Are you using water from a water softener? no
Has plant been recently pruned, cloned or pinched? No.
Have any pest chemicals been used? If so what and when? None whatsoever
Are plant's infected with pest's? None.

Have you scoped for bugs? Mainly russet mites/broad mites

Could be a pH issue as well. If symptoms went away for a bit, that's probably your culprit.

Thanks for your answer. I checked and found no sign of bugs. Actually as I suspected it was due been root bound, I transplanted yesterday and today the plants are looking healthy again.

looks like the soil is hot. keep them from drying out too much and they should grow out of it. you using led bulbs with the top cut off? get the plants real close then so they can get bigger and use up all that food they're choking on.

I think you might be right. It would explain why the issue goes away with the transplant, giving them a thorough watering when they go in the new por.

I'll start watering more often and check the run off EC.

And I'm still having this trouble. I even tried flushing the soil, but it didn't help. What I believe now is that it could be a mg deficiency, since my tap water is 20 ppm, just yesterday I gave them a watering with some epsom salts and seaweed extract. Let's see how that goes in the following days.

Ph is by far one of the most critical aspects of growing, not knowing what your ph is you could be locking out a specific nutrient without even knowing it.

For example if you have a MG deficient plant and try to correct it with a large dose of MG but your ph is off the plant will not be able to uptake any of the MG.

If you were thirsty and only boiling or frozen water were available that wouldn't do you any good.

Ph problems often start with twisted leaves and progress into phantom deficiencies. Leaving you chasing your tail trying to fix a symptom and not the problem.

Check the runoff e.c and PH

Check the ph of your water source.

Good luck!

Thank you guys in the end it was a pH related problem, I corrected it and the symptoms went away in a few days.