I believe there are probably two factors which over time might have a cumulative effect of dropping your pH.
1) In a single chamber model your probably dosing you tank w/ trace amount of CO2 which will lower your pH
2) Your adding lower pH water (the efluent) directly to your tank
My reactor has only been up and running about 6 weeks so I havn't seen a major problem w/ that yet. But just for safe maesures (especially since it's a 58 gal tank) I'm starting to use a comercial buffer. adding it extra slowly twice a day. pH is starting to come up a little but I've only been doing this for a couple of days.
My Ca reactor has been use about 9 months. Since my ph setteled at about 8.15, with a DKh of 12, I added a homemade Kalk reactor for my top off water. My Ph goes from 8.35 to 8.45. When I added the Kalk reactor, I slowed the CO2 bubble rate and increased the drip rate on the Ca reactor.
THe Kalk reactor is a used pretzel container (2 gallons?) with a maxijet pump on the bottomn which a timer turns on for 5 minutes a day. I have a dosing pump pushing water through an airline tube into the bottom with water leaving the top through another airline tube going to the sump. The dosing pump is controlled by another homemade item that turns on when the water level in the sump drops too low. Works for me