twisting leaves

any idea on this boys n girls?
thyre into thier 1st week of flower about 25 in tall
using bio bizz all mix soil niut wise on biobizz bloom 2ml per litre and 1ml biobizz topmax
600 watt hps about 20 in away from plants good air circulation
starting to get slight dis colouration in centre of leaves turning yellow.
feeding wise when i started them i over watered them and the leaves were curlin up on the serated edges now have gone the other way could i now be under feeding them? as thyre only having a litre every couple of days
any advice would be great thanks for your time