When my plants are that young i don't use nutes or if i do i use a seedling one part nute in low amounts.

can you post a pic of your set up ie How far the lights are from the plants/what lights your using.

also how good are your bubbles? if youve using them airstones for a grow before they get blocked up.

If your ph has been 6 or so this whole time it's not that and with rootroit cubes there more then enough nute for a seedling.

I would say the waters not aired enough to keep a seedlings roots misted above the water level and/or the lights too hot on the plants.

good luck

btw if ur using hard tapwater ( sounds like it ) not RO or soft water u souldnt need cal/mag at that young if not at all.