Sick seedling

FFHF can be hot, and your plants don't need a humidity dome. They are fine without it. Other good points brought up by numbers.

pic2 - burned down - to much light ; check with app - not give more than 150 ppfd to a seedling - you need just 10 Watt at 20cm ! not 150 or 400! water them with water (ph 6-6.3) , use a small lamp and feed them with water once a day a few ml to have the soil moisture but not wet arround it. they can recover in a few days. Even your windowboard would be ok in this state if there is some sun. less - not more ! they cant handle any burn or heat streas anymore, a mini foger would be great now to have arround 60-70% to unstress them from transpiration needs, to build new leafs and recover. And they cant handle any nutri stress now,too. Just Water. The soil has what she needs for at least a few days or weeks.

I can't tell if you cups even have drainage holes. If not, that's a major issue and you should make drainage holes

1. Overwatering (most likely primary cause) The soil looks compacted and too wet for a seedling that size. Overwatering suffocates roots, limiting oxygen and nutrient uptake, causing yellowing and leaf death from the bottom up. The stems appear thin and slightly pale — a sign roots aren’t thriving. Fix: Let the medium dry almost completely before next watering. Lift the cup — it should feel very light before watering again. Poke a few holes in the cup sides and bottom for extra aeration/drainage. You can add a small fan for gentle airflow to help the top layer dry faster.

The best guides to read are on cocoforcannabis.com. Least amount of bro-science. Even calls out some of the more egregious nonsense. As well as help you learn how to assess and discern from anecdotal nonsense and useful information.

Looks like several possible problems... First, let's tackle proper watering habits. 1) fully saturate -- anynone that says differently is retarded. If it is 'soilless' you also must get a minimum of 10% running out the bottom. That is waste water. don't let it sit in its own piss. That water is fine for an outdoor garden, but not a potted plant. 2) wait for appropriate 'dryback.' -- you can discern this by weight or depth of dryness. In a soil, let it dry about 1" deep before repeating. there is some leeway here, but stick to otrthodox until you can think on your own in regard to adjusting wet-dry cycle. If it's coco, it holds muchless per volume and you re-irrigate when the top starts to change color. The type of medium does impact this a bit, but if you go by weight, you'll quickly become familiar. if "soilless", you must also fertilize ever single irrigation with a full, well-ballance diet. With soil, how often you fertilize will depend on how concentrated you mix as well as the soil products you use. Over time you need to supplement what gets exhausted. This is a trial and error learning process... if systematic, take notes about the deficiencies you see and when you see them... can better adjust your fertilizing ramp up next time. Other note that is relative -- soil constitution. You have some sort of wood chunks and minimal perlite. Better to go with perlite or vermiculite to improve drainage than large wood chips. that stuff can rot and fuck up the pH or grow useless mushrooms in your pot. in a heavy soil, you want 50% of volume as perlite or vermiculite or similar.... in coco, which holds less water, you only need 33% of volume... when each is mixed to these rations, they have the same drainage and aeration properties, so there is no advantage to one or the other in this regard , if consitituted correctly. One of the ridiculous myths of coco's ill-perceived superiority, lol. ----- Pic #3 - exhibits signs of receiving too much light. There is little to no spacing between the growth nodes. Sounds like you've already reduced the light. The resulting node spacing of new growth will help you adjust light intensity going forward... once mature, it'll be consistent, but until then, go easy and ramp up as you see stretch. Take notes on timing and you can better adjust next time. This is an ongoing theme, as you can tell. ----- The yellow cotys this early and other issues seen on real leaves means that medium doesn't have much nutrients in it. you likely need to start fertilizing. ----- ph-balance anything you dump into the pot -- after any fertilizer is added. in soil - 6.5-6.8 and soilless 6.0-6.2. That really has more to do with the typical fertilizers involved than specifically due to the medium itself. If using soilless/hydro products, shoot for 6.0. Whit disitilled vinegar (5% acetic acid) is cheap an readily avaialble. Likely have some in your home. Get some 3-5mL pipettes to easily dose it out per gallon mixed. Any base or acid will work well. don't overpay for "ph-up" or "ph-down" products... end up paying 10x for the same shit... citric acid is a common ingredient on the ph-down. --- strongly suggest you use the smaller light early on... after you pot-up and the plants take up more space to warrant it, bump up to the 400w at that time. It'll be easier to handle, probably save some watts, too. Anecote: i've got some sprouts right now. The 150w XS1500 light is 20-22" away and at about 40%. I'm just trying to cover a 10x20 tray properly... a smaller area can use even fewer watts and get the light much closer. I'll adjust based on what i see... if they stretch, i'll increase light a bit... if they stay too tight and small, i'll reduce light a bit.... plant behaviour dictates everything and not the other way around.