It won't work. The reason is the very slow response of the drop checker to CO2 ppm changes. This would result in overshooting on CO2 to the extent of killing fish, then undershooting by so much as to trigger algae blooms. It is possible to make a drop checker with a gas permeable membrane separating the 4dKH water from the tank water, with a very tiny pH probe to measure the pH of the 4 dKH water. This greatly improves the response time of the drop checker. But, it is very difficult to make such a drop checker and keep it working, so the effort far exceeds the benefits.