Critical Purple Auto

They are 21 days old from seed to soil. Everything good until this week. The only change I had was water sources, I had to switch to store bought spring water with the freeze. I have switched back and not sure if thats even related. I think this is potassium deficiency??? I'm using ocean forest and have not added anything until last night. I gave them a worm casting tea in hopes to help them out. Any advice appreciated. I'm scared to do to much at this point, don't think ocean forest they ready for a feed yet.