pH problem

I have 6plants that are 3weeks old under 400w mh which is dimmed down to 75% and about 2feet from canopy.temps/humidity usually 75-77deg/50-60rh.using ffof soil haven't added any nutes other than molasses.plants started yellowing and slowed growth.my tap water was 8.5ph so I adjusted pH 6.5 and runoff was 5 so still adjusting but one plant showed nute burn on the tips of new growth after the adjustment.im only curious what would cause this after the fix as it wasn't burning before just slowed growth?

Do you have picture ? I'm no pro but cal/mag def could also look like nut burn.

I could take a pic but cant see much now I just bent her down to start lst but It very well could be cal/mag but using ffof I thought it was premixed with plenty of nutrients and I just transplanted it from solo cup to 1gallon pot at 2weeks..it acted fine other than yellowing due to pH being off then I fix pH problem and it acts like soil is to hot but it didn't burn none the prior 2weeks..just stumps me,no major issue cuz I know it'll come right out of it,just curious for more knowledge..happy growing

The 2 in bigger pots are 3weeks old,I fimmed then bent the one in the back yesterday and just bent the one in front today..worried bout fimming it cuz it took a lot of stress already and I have my worries that it's an auto but the others in solo cups are 2weeks old bout to transplant when soil drys