One thing though, if an aquarium were getting its CO2 from only atmospheric exposure (ie, surface exchange and bubbles from an air pump), would the gain and the loss not be pretty much equal?
Almost none of the gas in large bubbles end up in the water. The large bubbles burst at the surface of the tank. As these bubbles rise they are dragging some water with them, bringing it to the surface, improving the vertical water movement. The turbulence of the large bubbles bursting at the surface is also increasing the surface area of the water, so more gas exchange is happening for 2 reasons:
a) CO2 depleted water from lower in the tank is being brought to the surface.
b) larger surface area.
So, comparing 'no water movement' to 'large bubbles' there is a net increase in CO2 for the plants.
But in this tank there is already quite a bit of water movement from the filters. So the increase in vertical water movement may not be needed.
When the bubbles are being broken up this way, the Koralia is chopping them into little bubblets (if there is such a word) AND is sending them horizontally through the tank.
100 little bubblets have more surface area than one giant bubble, so there may actually be some gas exchange happening while those bubblets are in the water.
Also, little bubblets may stay submerged longer. Maybe not a lot longer, but some may be getting caught under the leaves and so on. The whole bubblet might end up disappearing!
I am not sure how tons of little bubblets compare to the same volume of large bubbles when it comes to surface agitation. If each bubblet creates a 1mm wavelet, but each giant bubble creates a 5mm wavelet plus some splash... but there are a lot more little bubblets...