bad quality airstones

[Bullfritzewt]

Literally experiencing this problem for first time in 15 years but after running tests with diff air stones in which used glass beakers for my study I made 2 beakers with an air stone. One with pH 5.8 distilled water the other with pH 5.8 water with 3 parts advanced nutrients at 4mL/L. I also had a control with no stone in just distilled pH 5.8 water and a fourth with no air stone and 4mL/L advance nutrients 3 part base nutes water at pH 5.8. I tried this with 3 different brand Amazon purchased air stones and I found pH change occur only in the beakers with pH5.8 with only 1 of the 3 air stones in 5.8 water with  the pH in those beakers rising to pH 7.0 over 12hrs and the water turned cloudy white/gray. I hypothesize it is certain calcium or other mineral containing airstones manufactured in China that react with the phosphoric acid used to adjust the pH this neutralizes the acid over time. Because in my test it never went above 7.0 and the other airstones stayed within 0.1 of the original pH over the course of days. I say this is a matter of poor quality airstones. Because these were in fact the chaired 3in air stones I ever seen in my life.