Transfer from soil to hydro problem

I transfer my plants from soil to hydro about a week ago. Transfer went well with exception of leaves are yellowing. I am using GH feeding schedule in the "grow stage". (3) Grown (2) Micro (1) Bloom parts. PH is between 5.5 - 6.0 and PPM 1200-1300. Plants are in DWG 5 gallon buckets. Lights are (3) 200 watt CFL 6500K. 18/6. In the process of upgrading to 400 watt MH. Mostly the big older leaves from soil growth are yellowing. Is this a nitrogen deficiency from soil to hydro?

What can I do to stop the yellowing of older leaves from soil growth.

Welcome Mama! Yes, that is exactly what N deficiency is, yellowing leaves, especially the lower, older fan leaves.

Can I stop the yellowing. Or just let it ride its course.

Extra Nitrogen will stop the yellowing. Without the N that it needs it will be stunted and might not survive, depending on how bad it is.

make sure your ph is in the right range to allow for nitrogen uptake, to speed the process you could drop it a little lower to maximize full nitrogen uptake. Trace elements also facilitate in the uptake of nutes so make sure you are supplying them