Unstable pH

For years I’ve used food grade citric acid to lower pH as my water source approaches 9.0. After some reading and testing I see that this is unstable - within hours of dropping it to 6.5 or so it goes right back to 9.0.

I can inagine that such a short stability window causes problems even if poured onto the plants (I grow in soil) immediately but that’s probably a complicated discussion. Are other approaches, liquid ph down, more stable and if so, which ones?

Phosphoric acid is often used in commercial PH down products, it works well.

You've been growing for years, how are your results?

If you've been using citric acid, and it's been working, do you really need to change anything?

I'm also using phosphoric acid.
If I have anything left in my water tank I usually check ph again before using it and adjust it again.
I haven't checked it after only a few hours, so I couldn't really tell you how long it takes before it goes up again, but it does.

I tried citric acid for a bit, for me, it was ineffective.