pH going up?

i'm having a problem with the pH of my nutrient solution:
i'm using tap water with a pH of 7
when i prepare the solution, i adjust the pH to 5,5, and when i check it after a few days in the drip system, it's gone back up to 7-8
is this normal? how do i keep the pH around 5,5??

oh, and i'm using the dutch formula advanced hyroponics stuff

Bump, i dont know the answer but i do hate ph problems.

the reason it goes back up is because tap water is highly buffered(stabilized) with calcium and magnesium. It will take an incredible amount of ph down to stabilize it.. so the solution is to use purified water.

Snake, any pics of your signature grow, "6 strains, 1 600w light, 1 meter squared
9 plants, 1.93 lbs?"

this is my old cab:

https://www.rollitup.org/grow-room-design-setup/80220-new-growcab.html

i am actually starting the exact same grow again today and will make a thread with pictures this time.. same six strains. Info is on the very last page.