That's what I did kind of too. I filled up a 710 ml bottle with gravel on the bottom and sunk it to the bottom of my sump. THEN I threaded my air hose into it. NOW I have a half filled bottle of CO2 dissolving gradually and excess just drifts out if it gets too full. Still, I'd use a limewood diffuser if I had one.
It doesn't even uglify the tank because it is inside my stand.
Only downside is that with that little surface area not much CO2 will be dissolving compared to the rest of the 18x36 tank and the 12x30 sump. ANYWAYs... it might work but not near as well. If you collect CO2 on a surface WITH an airstone too it will dissolve a bit better and less will be wasted.
My tank sucks now
Fraternity of Dirt # 22