Diffuser / Reactor

A diffuser or reactor is used to dissolve the CO2 in the water.
The most trivial form of a diffuser is simply to trap the gas inside an upturned cup, letting the gas to passively dissolve to the water.
There are a few other kinds of dissolving CO2 but the most effective one is using a power-head to mix the gas with the water inside a chamber filled with bio-balls or some other media. The chamber is placed outside the tank, allowing for a more esthetically looking setup.

Reactor chamber - placed outside the aqariumReactor chamber - placed outside the aqarium