Alkalinity in hydroponics

I have well water.. i will give example

I can get 5 gal of just water my ph is 6.7 my ppm is 300 before I do anything. I add 2.5 tablespoons of ph down ph 5.8 let sit for 12 hours and ph is back to 6.7 and now ppm 400. I can do this every day of the week.

My alkalinity in my water is over 400 ppm.

My ph keeps swinging up. I am add a lot of ph down every day so inturn my ec and ppm rises also. How do I lower alkalinity? And if I do will that help stabilize my ph. Plants showing a deficiency that I can't correct. Assuming it's a lock out do to high alkalinity.

Any help would be greatly appreciated.

Thanks in advance.