Tap water pH adjustment using citric acid I've lowered tap water pH from 7 to 6 using citric acid. After couple of hours pH raised again to 7.

What is the reason for this raise? Citric acid overcomes waters buffering  feature then raise the $\ce{H+}$ concentration in water. Why pH raise again?

The pH was measured using a pH meter.
Maybe citric acid have had a negligible effect on the $\mathrm{pH}$. Everybody knows that in contact with air, pure water is carbonated by $\ce{CO2}$ from the atmosphere. And the $\mathrm{pH}$ may go down to $5.5$. Later on, the water may loose its $\ce{CO2}$, and the $\mathrm{pH}$ goes back to $7$.