Using a lot of ph up

Seems like everyday I'm adding ph up. I start at 5.8 24 hours later it's 5.2, if I let it go 48 hours it drops to 4.8. By the time it stabilizes it's time to change res. any suggestions?

Root rot can cause big pH drops

It's not root rot. Roots are healthy and white, overall the plants look really healthy.

I switched to a concentrated ph up and down and it made a big difference. They do not break down like regular up's and downs.