Holy crap! I’ve been dealing with the same issue, and have also been using cheap blue aquarium airstones. I just filled a small bucket with fresh nutrient solution and stuck an open-ended airhose in it to let it bubble overnight, and plan on checking the pH tomorrow. I also put four of the aforementioned airstones (just by themselves, no air) in a bucket of pH 7.0 water that reads 25ppm, and will test that tommorrow as well to see if pH or ppm have changed.
I am very much looking forward to seeing if there’s any useful information to come out of this.