DIY CO2 and pH Problem

Thom Castrovince had a question about why his pH hadn't dropped
in spite of CO2 injection:

You should check that the CO2 reactor is actually churning CO2 and
not air bubbles.  If you are not using any aeration that might be
driving off the CO2, then a leak in the CO2 system or the reactor is the
first thing
that comes to mind.  I am using an airstone tucked under an
ornament that traps the CO2 with my DIY rig in a 20 gal. tank and
the pH dropped almost immediately (well, overnight anyway) from a
high that was off scale (which I attribute to biogenic decalcification)
down to about 6.8.  In fact I am concerned that it will drop too low.
Try using an airstone just to see if your system is pressurized and
that the CO2 is bubbling vigorously, then leak check the system
using the Trent Honea method... detach your reactor/airstone, attach
an airpump to the airline tubing, and let her rip.  If the pump sounds
like it's straining after a few seconds, chances are your system is
leak tight.

The other possibility is that your gravel is contributing to the
alkalinity of the water.  This would only happen, as far as I know, if
the gravel were composed of crushed coral or shells.

Sorry, but those are the only things that I can think of that it might be.