PH CONTROL in hydro

Good morning all,
I am having PH issues. I am running a rdwc system that i built. 25 gallon. 4 plants and 1 res bucket. Medium i am using is hydite . Water is at 68 degrees. Advance nutrients @half strength cause my girls are still little. Ph started at 7.1 so i dosed with white vingar droped ph down to 5.9. Next morning back to 8.0 dosed again. Same thing every day for the last 4 days. Any ideas on what is causing the ph to climb back up ?

Vinegar is not stable. It will only reduce ph temporarily.

I guess i need to quit being cheap . Buy the good stuff. Thanks

This chart was helpful in fixing my PH problems.

Its strange we buy good tents lights fans and such, then get cheap on other things. Like seeds and such