Help me diagnose please leaves turning brow dont know the cause

[Grow9011]

Hello.
This is my second grow.
The first one i finished around 3 months ago with a similar setup. I have a 120W led DIY panel witch worked fine on around 5 grows so far (not mine). On that light i added around 14 11W lights with warm white and 4 cold white 15W lights.
After i switched to flowering i only kept 2 blue lights and the rest were normal warm white lights.
The problem i have now is that the leaves started getting brown first on the tips. i thought it was too much nutrients. Then i watered it with only water ( i try to keep the PH in the 6-6,5 range with the digital ph meter but sometimes when i add nutrients i works strange so i check with the ph papers). After that the edges started getting brown. Then i saw it was too little "P". After i added nutrients to the wather the problem stopped.
Then i had a cycle with watering and nutrients and that ended well. But now 5 weeks and 5 days after the flowering started the leaves began to change color rapidly. Getting brown spots, turning brown ad dying.
I turned off 10 lights now to rule out light burn.
I started looking for bugs but i dont see anything.
What should i do.
Thank you for the help in advance

[Sheesh]

Hey grow9011, tough issue you have there. I wish I could tell you exactly what is wrong but I'm not sure. In some ways it looks like russet or broad mites but they don't look shiny to me. Check out this article and see what you think. https://www.growweedeasy.com/cannabis-broad-mites

The leaf you're holding looks like something is eating leaves but I didn't see a lot of bite marks on the rest of the plant, do you? With all the symptoms I'd guess some type of mite issue. The only other thing I can think of is root rot but that is more likely in hydro. Any bad smells coming out of the roots?