Seedlings weak and won’t grow

if they just sprouted in those pictures, relax. they'll take a minute to stand up and open up etc... use appropriately sized puts in future. it's difficult to water a big pot / tiny plant context. potting up does not "stress" a plant. if you didn't water deeply, that could be a problem. make sure it goes all the way down next irrigation. otherwise you are training superficial roots. seeds don't need nutes, but coco is devoid of nutrition, so fairly early on you must provide 100% of needs. always fertigate, always get 10% runoff, minimum. this eliminates risk of buildup and any symptom you see is 100% about adjusting the formula. Makes diagnosing easier, too. 600-750 ppm well-balanced -- based on calculation from gauranteed analysis lable and dose used and not some TDS pen conversion that is entirely inaccurate to measure ppms.