Anyone know what this could be?

Also re pot into black pots or cover the white pots with something i should imagine light could penetrate polystyrene pots roots dont like light

Your bottom leaves could have been touching soil when smaller also check your soil if its for seedling it will be pre fed and won't need any feed for 2 to 3 week u shouldn't need cal mg this early on .what watt light are u using and how close ? What temp is room

Ur good bro all the new growth looks good might have went a little heavy whit nutes in the beginning.I wouldn’t worry about it now…it’s very hard to have a pot root bound I never blame that for any problems I heroes 3 ft plants in a 2in rockwool cube nothing is root bound you just have to water more frequently…I keep seedlings in 1in cubes for weeks before I pot them ck out some of my diaries…good luck with the grow

Burning is from unnecessary feeding, stunting is from being in ridiculously small cups with no room for root development. The commercial soil you are using will have enough nutrition for seedlings for 3-4 weeks without the need to give anything else. Water quality is not the problem, any water fit for human consumption is fine for plants, chlorine and all. Don't worry about the humidity, it really isn't that vital and low is better than too high. Black plastic pots about 4 inches across and 5 inches deep are ideal for starting seedlings and will provide enough room for 3 weeks of root growth without stunting the plants, by which time they should be ready for transplanting. The best thing about decent black plastic pots is that they can be washed between cycles and with a bit of care will last many years, reducing garbage to landfill and making environmental sense. A plants roots will be roughly twice as long as the plant is tall, meaning that the roots of your seedlings in those small cups are running out of room very quickly, leading to the stunting problems you are experiencing. For pictures of a 3 week old seedlings' root system, which may help you understand this concept better, go to my diary "cannabis conundrums" and in week 7 you will find the pictures, along with some other interesting pictures of weird and wonderful things cannabis plants are capable of! At this stage, I would be transplanting your seedlings into much larger pots and not feeding them anything for at least 3 weeks after this, and when you do start feeding, start them on half strength nutrients, as this is usually plenty strong enough. Remember too, it is very easy to correct a slight under feeding, but very hard to rescue an over fed and burnt/damaged plant! Hope this helps, Organoman.

The tap water here doesn't have chlorine in it, humidity could be the problem aswell as we dont have a humidifier in there yet (when they were really young we used cling film to keep the humidity up). Thanks for the replies! If this run will die out the next one will be in a diary 😄

Dehlorinated water you need to use, tap , especially now in cities uses chlorine versions wich evaporated only in 72 hours, not previously used versions, for which even 24 was enough. My wild guess would be there. Humidity maybe, but most leaves are ok and healthy, no overwater or wind damage. Excessive nitrogen could cause it too, but it doesn't look like ... Hope you'll find answer ! All the Best !!!

Premix is a high quality slow release (6 weeks) fertilizer for young plants in general. Containing: 50 - 250 mg/l N 80 - 300 mg/l P2O5 100 - 350 mg/l K2O The leaves started wilting and then drying out from the tips. At the start they might've had a little too much water, otherwise watering when the top soil is dry. The brown spot on the top leave was probably due to a water drop but the lower leaves are something different. The tap water is Ph'd down to ~6.2 (before adding CalMag and Micro but only got one feeding of a fraction [1/4] of the recommended amount per liter) Lights have been lowered to about 25 inches and some holes were added to the little pots to keep the roots aerated. I suspect that the roots could have been drowning/suffocating for a bit since there were only a few small drainage holes in the cup. But I dont know if this is what suffocating roots look like.

whats in the premixed soil?? or what brand and bag is it? where the spots deformed before turning brown and crispy? like over watering issue? how often do u water. Do you keep the leaves dry when you water? also I've not yet seen acidic tap water, I'm used to hard water in the alkaline side of the scale. Is the 6.2 PH after adding calmg and micro? Your could also lower your light a bit. PPFD map at 18" has low 250 high 450 for umol/m2/s 25 inches would be better working your way down to 20" or 15" Good luck.

No diary its hard to say. It kinda looks like the start of WPM. The other dark spot on upper leaf looks like maybe had water on it during lights on.