Can anybody tell what this is ?

Hi this is my growroom where i have 23 young plants of various type : Super lemon haze, g13 haze, bubba kush, white widow, moby dick, big bud. They are rotating under 2 lamps : 1 MH 400W, and 1 MH 600W. They are all from fem seeds that were popped the 1'st of april. I use 5 litre pots with biobizz light mix soil. I started to feed em since 1'st of may, once a week, with a quarter of a dose then going up till 1 full dose the other day. When i feed, i give every plant 1 liter of water with this nutrients: 5 ml Canna terra vega, 2.5 ml cannazym and 2 ml n27% (nitrogen boost). 10 days ago some of the plants started to get a bit yellowish and the leafs are drooping. 2 have died allready. I kept the pH of the feeding solution at 6.0-6.2 but i have been told that is not right so i flushed all with 6.6 pH water ( that was 4 days ago), then the last feed i gave them was pH 6.6. Room temp with lights on 26-27°C, with lights off 22. I have no idea what this could be about, please help. The plants will go outdoors in 7 days, i hope they make it till then.

nutrient burn

r u sure dude ?

yeah i looked at some photos and i think that might be it thanks

almost positive, i would flush them with some ph'ed water

Looks like nute burn to me. I'd flush them with water. Make sure you over water when flushing so the extra salts wash out of the soil. The roots will take a while to recover as if your leaves are that burnt they're probably worse off. Go a few weeks with just PH'd water and possibly some CaMg+ depending on your water source though from the look of your stocks I don't see any signs of deficiency.