pH drop---what's going on???

Hi there,

I have a tank that's been set up since Feb with CO2 injection. Everything was humming along fine and the pH control was working great keeping me at 6.5 by addition of compressed CO2. KH=2, and GH=4-5. I recently left for a week and was horified to find the pH meter reading 4.5 when I returned. I watched things for a day and found that no CO2 was being added and also checked to confirm that the pH meter was still properly calibrated.

I've performed a 30% water change which brought the pH back up to 5.9, I've not had a chance to measure the GH and KH since I returned, but I'm wondering if anyone has had similar experience and what the root cause might be.

Thanks,
Caywood