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)

My tapwater (from a well) comes out of the tap at 7.9ish ph. After adding 1/4 tsp of phDown per gallon of water, the ph is about 6.9ish. Adding nutrients to that, (Fox Farms) my ingoing water is at 6.3ish. Everyone's situation will be different, so do not use my numbers as a hard and fast rule, but as an example. Adjust to suit YOUR situation.

For instance, my water comes from an underground aquafir, through layers of magnesium/limestone bedrock. the minerals in my water want to keep bringing-up the ph. (I don't R/O my water) So my quantity of phDown might be a bit of overkill for someone else's 7.9 water that's unaffected by minerals.
Perhaps intimidating, but not really rocket science, lol.

Any of that make sence?
Rusty Trichome Reviewed by Rusty Trichome on . Yellow crispy fan leaves on 4 different strains.Deficiency or hard water? Hello people.It's been about 10 days I have this problem and it seems it's getting worst gradually.I'm growing 4 different strains:Masterkush, Super Silver Haze, Ice Cream and Blue Cheese. Here are some pictures: 279005 279006 And the most affected plant, masterkush: 279007 Rating: 5