Northern Light + Watermelon

Day 8 (09/12) - First day of second week First week was perfect. All the plants grow a lot and seems to appreciate the nutrients. For now the lamp is at 80% lightness, temperature in the box is 29C during the day and 26 during the night. PH soil si perfect too. From now i ll call N1 and N2 the two Northern Light and W the Watermalon. Day 10 (09/14) - Yesterday I started with LST. The plants reaction seems to be good Day 13 (09/17) - in the last 3 days i had some problems. The oldest leaf of N1 and W turn Yellow and burnt. I think that maybe i feed them with to much nutrients and i stack the roots. Last night i tried to wash the roots with pH 6,2 water. Lets see if something good Will happen. If someone have some good tips about or maybe think that im wrong pls feel free to tell me. I ll keep you update :)