CO2 loss will take place starting at the surface drains, within the tubing and in the sump. Is there a drip plate above what looks like bio balls in the sump? Perhaps you can place a sheet of filter floss or thin sponge below the plate and above the biomedia to reduce the drip/splash. That is a lot of bio filtration for that tank (I had a similar sump on a 55g once). Replacing some of the plastic media with sponge mats might reduce the amount of air exposure and CO2 loss. Just a suggestion.
I believe the plants in the tank are not too demanding CO2 wise.