Can anyone tell me where I went wrong?

I can almost guarantee that you didnt water enough. Your humdity and temps r good enough and even if the hum was super low or high, it wouldnt cause that. Please dont listen to the guy saying coco is the worst. Coco is good for beginners because its hard to overwater. The reason hes saying is salty is because he prob used brick coco, brick coco is processed near the beach sometimes where its very salty. And those types of companies dont want to waste money by properly washing their brick coco. Most bagged coco is washed. And trust me, if your plant was healthy in veg then it wouldnt cause this now. If anything, if your coco was too salty then it wouldve killed your plant way before this in veg.

You need to practice lifting your pots before and after watering. You can water alot in coco and perlite as long as you lift it up and get rid of excess runoff. I can tell from your video that your coco looks dry, maybe bone dry. Touch the top of your coco and if its super dry then its because you didnt water and killed it. Either ways those leaves that r dying are gone. It may feel wet but that might be because of your humdifier spraying directly at it. Also those leaves are wilting, wilting leaves can feel almost wet but theyre just dying. Send me some photos of the top of coco, next time just message me.

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