Please help some of the ladies are getting those yellowing & light green edges on the leaves and tips

[Peterpangreen]

Hi guys,

Asking for some help, got a 25 plants 11 strains in 11 liter smartpots in lightmix.
About 3,5 weeks old.
EC 1,5 / 750ppm
Using silica, calmag, plagron grow, plagron roots.
It started with 3 plants and it is getting worse.
Temps at night are low at 17C / 62F.
Here some pics of the yellowing issue:

 

[Green thumb lou]

Check your Ph. That’s important for your plants to feed

 

[Peterpangreen]

Check your Ph. That’s important for your plants to feed

Yes I always do, got it between 6.0-6.5