Planted Tank Enthusiast
Join Date: Feb 2017
Location: Lindenhurst, IL
Carbon dioxide is the densest of atmospheric gases, so you don't actually need to completely seal a sump to keep CO2 above the water. It's not like you are really going to be able to seal it to the point of pressurizing it anyway. Just think about the gas blanket that forms I've dry ice in a container (there is a temp difference there too, but it illustrates the point).
Unless the sump is a wet dry, you should probably be more worried about the overflow causing off-gassing.
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“Science, my lad, is made up of mistakes, but they are mistakes which it is useful to make, because they lead little by little to the truth.” -Jules Verne