It happens every now and then, fortunately rarely, to find on a few lesser known some indication of the Speaker of CO2 in the aquarium ... and their placement unless indicated: immediately below the entrance to the internal or external aquarium filter!
Usually we talk about how to lay out, materials, prices, their use in the aquarium to spread in the best possible carbon dioxide in water, dissolving it in fact.
And in almost all sites are also shown the correct places:

1. at the bottom, because the bubbles are able to make the longest path possible to arrive at the water surface and then disperse unnecessarily
2. under current, so that the bubbles of carbon dioxide walk as much as possible in the horizontal and then employ a lot of time to arrive at the surface ... maybe not coming at all because shrink hand hand until it disappears ... excellent!
3. front of the filter output external or internal does not matter, then down and "current", so the micro-bubbles if they go around and slowly dissolve completely in water
4. in output of the filter and not under, but just in ... letting the bubbles at the end of the tube, so as to "shoot" even faster and taking advantage of the vortices that are formed to further chop

E 'instead to avoid the location below the entrance of the filter, where the filter takes the water and the door to the sponges / wool and then to the materials for biological filtration, because in the filter there are the aerobic bacteria (Nitrosomonas and Nitrobacter ) and therefore they need oxygen to fully play their work ... certainly not carbon dioxide!
So much so that in densely populated aquariums and devoid of a plant, eg the ones dedicated to African cichlids or large cichlids Americans, is used to insert an oxygenator entrance of the filter.