Yellow crispy fan leaves on 4 different strains.Deficiency or hard water?

[stefanosstef]

Proper ingoing water ph, in peat-based soil, is between 6.3 and 6.8 if the soil is already properly buffered. You didn't mention brand or kind of soil, but check the bag to see if it says it's ph balanced. Since you mentioned that the soil is "slightly fertilized", it might be that you're adding too many nutrients before the existing nutrients in the soil burn-off. (get used-up)
Which specific brand and type. (Miracle Grow 6 month, Scotts 3 month, Fox Farms Ocean Forest, SuperSoil...?) Is it an indoor potting mix? Just curious, because outdoor mixes tend to be virtually unbuffered. (ph is too low ofr indoor cannabis)

Unfortunately I don't have the soil bag or remember the brand but It wasn't big brand, that's for sure.I think it was german...
I'm not sure about the ph, I think it was 6-6.5.If it was unbuffered or with much fertilizer wouldn't it damage them when they were seedlings?When I fed them they had 34 days in this soil so the nutes should have been gone.Last year I used the same soil and fed them earlier with no problems.

The liquid ph test kits are better, more accurate than the strips, IMHO. Possibly cheaper as well.

I also ordered a kit containing liquid ph test kit, ph down solution and ph up solution paying 26€ for all of them.