Yellow lower leaves

I'm brand new to DWC and less than 3 weeks into my first grow.
I'm using nutriculture double bubble boxes that have a drip ring and lower nutes tank.
Running 650 watt light set at 300 watts on the dimmer at the moment.
Daytime temp 80f nights in 60's and around 45% humidity.
I put these babies in the system at 9 days from cutting taken and watered with 1/4 strength nutes to start.
After 1 week I noticed the lower leaves turning yellow and progressively getting worse.
I followed some advice and changed the water and nutes and have been using plain ph6 water for 3 days but the plants aren't looking any better ???
They wilt a very little but just don't look lush and green.
I water these for 15 mins on drip ring 5 times during daytime lights on.
Can somebody please help as I'm really worried about this.
Thanks for anyone who takes time to reply

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I planted in the system 9 days after the cutting was taken and have been planted in my system for 10 days.
My light is currently set around 20 "from plant tops.

Sent from my GT-I9300 using Rollitup mobile app

okay leaves are curling up means heat stress. and also looks like some nute burn.

Are you using rockwool? If so, check if it is waterlogged. My plants almost died because rockwool soaked too much water and was drowning the roots.

I'm not sure about the heat being too much as my ballast is only set to 300 watt and around 20" from the plant top's. I've put my hand under the light at canopy level and it feels fine.

Spanky.... They looked like the root riot clone cubes to me or a similar brand but deffo not rockwool. The cubes aren't particularly wet also, just a little moist at the moment.
The roots are definitely growing though as I just noticed them pokeing through the underneath of the tray (and weren't 2 days ago).

i ruined 3 autos and they looked similiar. id say nute burn and/or heat stress

Not good !! It doesn't seem to have carried through onto the other leaves apart from the lower. They are definitely growing still and I've flushed them for 5 days so I'm going to try them with 1/4 strength nutes again today. Here's a pic from yesterday