PLZ help!! Sick plants

My plants (babys) are sick, 3 out of 5 are yellowing and some spots have been forming on leaves. All the Plants are in the same soil. Useing a 400 w hps now,for last week. Have and tried a 400 w Mh but dont thik it works as well as HPS, anyway. The two that are doing better then the others are very dark green and leaves curl down like Rams horns. other wise look Ok. I also feed them full strenght MG all perpuse plant food 24-8-16. Plz help. lights are bout 2 feet above plant tops.