Phosphorus and PH drops in flower WK 4

So this is more to help someone that may be having similar problems than needing help.

I am a new grower and I am running my first grow in a homemade rdwc.
After about 3 weeks into flower I started having curled leaves and spots show up.
After a res change, PPM would drop for a few days and my PH would be stable.
About 4 days after a res change, my PPM's would start to rise, and my PH would tank.
Roots look and smell like the inside of a potato, so not rot.
I couldn't figure it out. I think I finally found the answer.

What I am assuming is happening is the plants are sucking the phosphorus out of the solution. Once they have depleted the phosphorus the PH tanks, PPM rises and will continue doing so until you add some P and or K.
Once the plant has used the phosphorus, the PH tanks as P is what is doing a lot of the PH buffering.

Now, this PH drop may only be Phosphorus specific. I imagine if the plant was sucking Nitrogen and running out of it the PH would rise once the Nitrogen was taken from the solution. Just based on the very little I know about cations and anions.

Anyone with more experience than I have thought on this? I will post pictures once I get to my main computer.

I know it was phosphorus btw, because it was only showing up on older leaves. Purple spots and curling. Burned tips.
In its early stages it appeared to be a nute problem because all that showed up first was burned tips. This made the problem worse as I ran my PPM's lower thinking I had nute burn.
I imagine a lot of new growers can confuse the first signs of a deficiency as nute burn.

Not sure if you are looking for help, or making a statement lol. Pics would help..you have alot more reading to do with regards to anions and cations..the things you are saying make zero sense at all!