Hi Jaap,
the device you are talking about is very interesting as it (is supposed to) fully dissolves all CO2 bubbles in the tank water. So you will save CO2.The bad side is that you have to introduce a big (and ugly IMO) device in your tank.
Ceramic diffusers are great but a lot of the bubbles reach the water surface meaning that the CO2 they contain is lost to the atmosphere. In that case you also have to introduce the device in your tank, but ceramic diffusers are aesthetically more attractive than the CO2 mixer.
Atomisers are even better than ceramic diffusers because they generate very tiny bubbles that travel into the tank following the filter current, and as a consequence a high proportion of the CO2 dissolves, but even with this system there will be CO2 bubbles reaching the surface and saying good bye. In that case the device usually is an "in line" one, that is, connected to the tubing of the filter system, so you don't have to introduce any device into the aquarium.
From my point of view these are the pros and cons of the three systems. It is up to you to how to solve the trade off. For a 20L tank I would go for the atomiser or the ceramic diffuser, and not for the mixer device that will invade most of the tank space.
Hope this is useful for you,
Jose
