Join Date: Mar 2013
Location: London, Ontario
The same phenomenon happened again. I did a 40% water change. CO2 was injecting at 2 bubbles per second, with the bubbles decreasing in diameter from about 4 mm to 1 mm at the top of the Hagen Ladder.
Within a few days, pH dropped from 7.8 to 6.8. Measurements in the morning, prior to lighting. KH and GH both around 100 ppm. The plants appeared to respond by producing lots of oxygen bubbles, including constant streams of microbubles from the Amazon Swords.
About a week later, even though the CO2 bubbles were still behaving the same way, I could not get the pH to drop below 7.4.
So, again, it looks to me like there is something in the tank (likely the gravel?) that progressively reduces the ability of the water to change pH overnight due to CO2. This appears to be only temporarily overcome by the addition of a large volume of new water to the tank.