OK... you are growing in 100% peat moss, under a 600w HPS, and using pH 6.4 water. Humidity 50-70. Temp day 25 C night 22 C. I think
@agedmaster has correctly identified that you are having leaf burn from the HPS.
I think
@Emilya Green has expertise with peat moss.
pH of peat moss is around 3-4, so needs calcium to bring up the pH. If I was to try to grow in peat moss, I'd probably add dolomite lime and/or oyster shell powder to the mix. I'd also add a lot of other dry ferts, worm castings, and perlite. But I understand you are using CalMag liquid when you water.