his problem looks very similar to mine, I used coco bricks, I then flushed them for 5 days, then give them half strenghth feed (following canna charts and less then it says), there has been improvement, but im not totally satisfied what the exact problem is
feeding with bio bloom and top max (trying to get them flowering feeding about 500ml per day)
just maid some fresh feed yesterday, ph for this is 8 and ph is 7 for water
my temps are way to high too, ive got a min-max thermometer and its recorded a high of 106.5f and a low of 73.6f this temp thing is starting to get annoying now
i have 600w hps light
4 inch fan pulling air out and 1 big oscillating fan and 1 small oscillating fan to circulate air
box is about 153 cm high 124 cm across and aprox 200 cm long