Brown spots on seedling

Hello, Did you check for pest? Small bugs can nestle on the bottom of the leaves, and make dark spots. Could be over nutrition,the leaves will be fat and quite dark most occasions. Give it a week of tlc,without adding extra magic stuff, it’ll turn out fine I gues

Could have been from a bit of water splashing on them when watering. Could also be if your lights are to close. Led lights tend to make the plants use a bit more calcium.

Good afternoon, it seems to me that you have a pH problem or maybe with the soil. It could also be humidity; in the photos, it's a bit difficult to identify. Water with regulated water, check the temperature if it's indicated. Hug