Need help deficity k or p?

If you can provide more information, like your nutrients, dosage, ph, EC, people might be able to give better help. It's hard to know for sure just off a picture alone

That looks more like an early nitrogen (N) deficiency rather than phosphorus. It’s common when the plant switches to flower it starts using stored nitrogen from the lower leaves. Add a bit more N in the next feeding, and increase your PK slightly (a bloom booster helps). Potassium deficiency usually shows with edge burn or rusty spots, not uniform yellowing like this.

This looks like its been happening over a bit longer then just the flip to flower. No diary so its hard to tell. Your going to be your best judge in this. Take what information people are giving you and see if it adds up. Tap waters Ca is almost always in a carbonate form that is not easily broken down and made available to plants so sometimes its best to supplement with a calcium product just to get it back to base line. Same with Magnesium in tap water. the Cl/flourdie is almost negligent and does not affect grows from my experience, some ppl freak out about this. I would first look into your feeding regeme and confirm its hitting all its targets for NPK and secondary macros CaMgS then move onto micros like Fe,Mb,Zn exct. https://420life.com/wp-content/uploads/2021/10/Marijuana-Leaf-Deficiency-Chart-scaled.jpg This chart can help a bit. but recongize it is lilkey a few things not just one.

They probably need a little nitrogen help, especially in the first try weeks of flowering!

Diaries really help to analyze what's going on so we have a better clue with what you have been doing and what could have started this. It's not always nutrient related when your plant is looking sick. This plant has been stressed for awhile now, the whole plant is yellowing.

are u sure his tap water doesn't have plenty ? ur advice could actually F up his plant... 👇😤

hey 👋 After flushing, be sure to use a flowering fertilizer, not a vegetative fertilizer, in smaller doses or 1 out of every 2 waterings. Also add 2 ml of CalMag per liter.

overfed flush asap and feed anew