Thanks for all of your suggestions.

The tank had been set up previously so I am fairly confident about the
behavior my non-co2 water values. (The little minnows I kept sadly
were infected w/ tb and died off. I took the tank apart and gave
everything a good bleach soaking/scrubbing followed by lots of rinsing
and decholorinating, a new filter pad and bio-wheel, and new gravel.)
The ph is 7.0 out of the tap, 7.5ish after standing (CO2 dissolves),
and used to fluctuate from 7.8 to 8.2 with the presence of a plant.
The kh had always been stable at 4.5.

So, you're right, something is either screwy with the gravel or the diy
CO2. I removed the CO2 for now and changed the water in the tank, and
am going to observe the kh/ph with just the gravel. I think I'll also
try the CO2 container test you mentioned. Hopefully I'll get some
clues!

I'm occasionally in NYC and enjoy the famously soft water. ((My
friends tell me it's the secret to those delicious bagels.)