Sera flore 500 reactor efficiency

Here's a general principle that will make any CO2 reactor more efficient: turbulence increases dissolution rate, NOT dwell time. So the Sera and other similarly styled reactors will always be poor because they are designed to increase dwell time, not the amount of turbulence. So, anything you do to increase the amount of turbulence in the reactor will increase dissolution rate.

Adding bioballs or other funky shaped objects in the reactor is one way of increasing turbulence.
And a large pore foam at the bottom or in the outlet tube of the reactor will help prevent, but maybe not eliminate, the bubbles from escaping without dissolving.

 

The impeller's purpose is the break up larger bubbles so that it is smaller. It does not create turbulence. The bubbles are spiraling because they are caught in the water's current, where there is little turbulence and relies upon passive diffusion to achieve dissolution.

 

I have an Ista Mix Max reactor. It's basically a cheaper version of the Sera. It too, produces very small bubbles in the aquarium. Not as much visually as a diffuser but, none the less noticeable. I just like think of it as a more "efficient" diffuser. The advantages are, that it is in the stand so, the noise is reduced, I don't hear bubbles hitting an impeller, and it doesn have to be cleaned as often as your standard ceramic diffuser. Since the CO2 is in bubbles, you do have to be mindful of the current in your aquarium so that the bubbles pass by your plants throughout the entire aquarium. I have my Ista powered by a separate water pump that is also in the stand. I did this because I found the Ista to be too much of a restriction on my canister filter.