The trickle filter has two water returns. The other return utilizes a Magnum 330 water return fitting. To provide surface turbulence, a Magnum diffuser was used to direct the return flow across the surface, producing ripples equivalent to the powerhead when placed at the surface. For tests without turbulence, the diffuser was removed, allowing the water to be directed towards the bottom of the tank. The tank is bare except for the equipment mentioned - no gravel, no livestock, no plants.
Lighting is room ambient.
The top is open. Note that GH has no bearing on the CO2 measurements, but a KH of over 3.5 is needed to ensure accurate CO2 test kit readings. A KH of 7 was selected to keep pH readings in the range of the LaMotte test kit. After letting the water equilibrate for one day we measured dissolved CO2 at 2-3 ppm. The CO2 remained about 2-3 ppm.