@Illyssian, so, it was partly the air pump. The air pushing through the stone fed minerals into the water. I knew something had to be getting added in from the air source. BTW, my answer in my first response (read it again) said the air pump was feeding calcium or lime into the water. The only dofference is the minerals weren’t in the pump but after it in the air line. Had you mentioned he was using an air stone, none of us would have been stumped.That was a sadistic game. Your suggested solutions were all misleading and you left out a vital piece of information. You should have given us 0 (zero) hints and just asked for opinions. 
@anon32470837, yes they can, but to you are missing the argument I’ve been making, aside from possible outgassing, none of your alternatives is going to continually change the pH, bringing it back to the same reading. Any minerals in the water have already made the pH difference. Something HAD to begetting continually added to the water to do that.
Edit: made additional comment.