Not quite. Look at my experiment above with the tap water that had been PH’ed down to 5.11. After adding the PH Down, the PH of the water immediately started to rise back up, and it continued to increase for 20 some hours - until it reached the buffer level of what ever agent(s) was in the water. It started rising slowly until the acid was almost all neutralized, then went up fairly fast until the buffer level had been reached.
In my case, I suspect there are microscopic calcium carbonate particles suspended in the water in addition to how ever much is dissolved in solution. The acid will neutralize the dissolved stuff fairly quickly and lower the PH right away, but it can take time to dissolve the solid particles. So its very possible for the PH Down to drop the PH but then get used up and the dissolved minerals that are still there start raising the PH again.
Also, with that type of stone - made from calcium based rock - the PH would have risen even with no air flowing. All it needed was contact with the water. The flowing air would have speeded up the process though, by constantly refreshing the water in direct contact with the stone.
I suspect something similar is/was going on with my lava rock and perlite. Perhaps they were packaged at some place that also works with calcium based minerals and there was cross contamination on the assembly line somewhere. My water is well water so you would expect dissolved and suspended minerals.