Nutrient Burn Or Deficiency Or Ph problem ?? Help

Hello, this is my second grow, using 450w led in a 2,5x2,5x7 with 4 Northern Lights Autoflower !!
They are at day 50, aproximatly third week in flower, seems to sufer hard from a deficiency or a ph problem

My grow room is at 26-28c and the rh is between 40-55%, growing in Pro-Mix soil in 5 gallon pots

Gave nutrients solution half strenght of Bloom mix from Green Planet !! Dont know if it helped its been 2 days i gave them
The problems started with the lowest leaf and came up to the head of the plant in one-two weeks !! dont know what to do !!! The tips are curling up also ( just started to curl up )

Except the two last Pics it is my plant that is the farest in the flower ( have the double the size of the other 3 )
It never had problems except now the top leaf are yellowing, light greening on the sides, and the tips seems nuts burned ( dont know if this is burned or not... )

Tell me if you need more info im kindda new in this forums stuff and in the grow thing

Whats your water like? Ph etc

Always give water at 6.2-6.5, it is not hard water ! never test the rinoff i should do it ! Mi TDS Meter seems veery wrong in calculating the ppm's so i wont trust what it told me...

Pretty sure it is a phosphorus deficiency, i mean it looks exactly lile the pictures on internet and the plant have the same symptoms as described on other threads! How can i fix this with my green planet nutrients ?