Hey there Saggitaur,
For future reference, TANKJR hit on both of your problems.
Seedlings need low level, blue dominate light for good root production. Consider using floros while starting seedlings or just rooted clones, then at 3-4 weeks they will appreciate the 400MH.
Plus no need to start nutrients too soon either, and when you do start go very weak, you can always go more if they show signs of needing more. But it is hard to un-ring the bell when you go overboard.
Both of these conditions set back the growth, and takes the plants a long time to recover when the conditions improve.
Keep it civil please, gentlemen. -StinkyAttic