with a kh of 4...that means you can run at pH of 6.7 (bare bottom value) and have about 24 ppm co2 but...
when your lights go off and your plants consume o2...your ph will drop due to a somewhat low kh value...thus the fluctuations you see...
i run kh at 6 and have my ph controller set at 6.85 and see a 0.2 pH fluctuation when the lights are out. not the greatest fluctuation but at least my discus are doing okay!
