Just opinion, but six weeks into veg and you're hitting them w/ 700ppm, maybe a little more sauce and light. 900ppm wouldn't be too much for six week veg plants. Are you supplementing ambient co2? Do you have fans blowing on them? Night temp? They look pretty large and it looks like you have floros, either you need a light with more penetration like an hps or mh, get your florescents closer, or clear out some of your old leaves.
Bring the light closer and get rid of the leaves that aren't getting any light and you probably wont have to worry about bumping up the ppms. You said they are mothers. So a few missing leaves isn't gonna shock them too hard. Its not like they are in flower. I usually bend the old darker leaves backwards and if they are no longer young an flexible, they snap. If they don't snap, I leave them. Get it "leave" them? I am so witty sometimes. I'm pretty sure this is not one of those times.
If you wanna spend some money then I'd add an oscillating fan, add some silica, supplement their co2, and give them girls some more light. Just opinion.
