Growing in fox farm ocean forest mixed with happy frog, It’s Strawberry Lemonade Photo by Barney’s Farm, i PH my water to 6.4 EVERYTIME, I also use tap water because my EC only reads at 134...And I’m about 3 weeks in flower
Does your tap water have chlorine or chloramine in it?
Are you measuring PH out?
What's your RH and PPM?
What lights are you currently using?
I almost want to say Calcium, Magnesium or manganese deficiency.
Edit:
https://www.mrgrowit.com/post/ph-fluctuations-in-cannabis-plants
These would be my guesses from the info you provided.