Hi Chuck,
I bet your well water is quite different than many other well users. Mine is extremely hard, high KH. So you have no carbonate buffer for the biologic processes to consume. I think folks will say the pH rises as it ages as CO2 is off-gassed. In your tank, the nitrification process is making it more acidic, setting up for pH crash eventually. You probably know all this, folks will correct me if I'm wrong. I think baking soda, sodium bicarbonate is frequently used to add carbonate. Also, I've heard of crushed coral, calcium carbonate, being used.