Curling leaves, brown tips.

Hi. I have got problem with my plants. They are wappa clones, 6 weeks in flower. They are in recirculate top drip system with 10cm grodan cube.I water 3x15min during light on. I use RO water plus CalMag with Flora Series EC 1.3 . PH is 5.7 . I use 400 watt hps in cooltube. My day temp is 22-23 C. Lamp is 25 cm above plants. I cheack with hand, no burn, little heat.

Keep your pH at 5.9 to 6.1.....Adjust daily.
What is the relative humidity?
The plants look like they are hot and starting to get nute burn.
Flush the cubes and dump and refill the res with fresh solution.

Hi. I have flushed cubes last week, nothing change. Ph Is stable, it doesn't change so much. I bump it to 5.9. They drink around 3-5l water daily. I top res daily. Ec every day goes down. Yesterday was 1.3, today is 1.22. They grow nice, now start filling buds. Today I Will put second temp sensor below lamp at canopy. last lamp on temp was 21C,rrh 60.That was at the corner at canopy level. All plans show this, nor only below lamp

Looks like potassium def.
Check this out 3rd post down.
https://www.icmag.com/ic/showthread.php?t=231387