Now if only I could take my own advice! I finally gave up on the PH test kit and bought a PH probe - a Milwaukee pH53. So, I calibrated it and tested the tank water. The reading settled down to 6.34 (love that extra decimal point). I took a tank sample and let it sit for 4 hours, sloshing it every hour or so. Read the PH and got 7.25. Not bad, huh? Then I remembered Tom's advice about the interference the ballast can cause. So, I turned off the light and retested the tank. Got 6.64! Drat! Why does fate work so that every damn mistake we can make always overstates the amount of CO2 in the water? Is this known as the 98th corollary of Murphy's Law?