What mgamer is saying, I believe, is:
Airstones(pumping air, not co2) are a bad idea for co2. These create surface agitation, which is bad for co2.
HOB's create surface agitation which is bad for co2.
Cannister filters, if angled the wrong way(i.e, towards the surface of the water), can create surface agitation, which is bad for co2
One step further, while effective, there are better ways to diffuse co2 than through an airstone. You're aware of this. However, sometimes you just don't have a choice due to space issues, so in your 5 and 10, you just might not have the space for another diffusion method.