I just solved a similar issue for someone about 2 hours ago.

What is the surface movement like in the tank and the circulation?
You should have some ripples on the surface, not a lot, but a good flow across the surface.

During the day, this will mix the CO2 better, and blow off any excess, so you will lose a little also, that's okay, it's cheap, at night the added current will add more O2.
During the day, the rapid pearling plants will add plenty of O2, the added current will help mix the O2 and the CO2.

They act independently, but due to the plant growth/pearlingm, there is a relationship.

You will also note a much improved fish health and plant health if you add the surface movement and the mist.

Mist is too intense for many set ups with the surface moevement, but even reactors have this issue also.

I think 6-7 x out 10, most folks have circulation issues rather than high CO2ppm issues directly, the CO2 is exacerbating the the low O2 effect on the fish and a little added O2 cures that right up.

Regards,
Tom Barr