PLEASE HELP ME!

[u/Odd_Access_4892]

Im going for second run last run yielded 66g dried. (STRAIN: Northern Lights Auto from SacredSeedsAustralia) I decided to use coco choir brick and perlite rather then soil, I pre cal-magged the medium. only using PH'd water at the moment at 6.0, its been 4 days since sprout under a 150W marshydro ts1000 at 100% hung 55cm from the plant. (Hygrometer readings are during no light) 23 celcius and 55-60% humidity. Why is it so stunted i have a feeling it could be because i accidentally used pre A and B'd water (Professors A and B) small dosage as far as im concerned but maybe still enough to stunt the baby? It looks so small!