Just checking. Do you have anything to diffuse the CO2, I mean, is it just coming directly out of the hose end or have you got something to difuse it - make smaller bubbles, as the smaller ones have a much greater surface to volume ratio and will disolve easier.? If not, can you pick up some permeable material from the aquarium supplier, perhaps some coral that can be placed over the outlet? Another option may be to run a long hose around the bottom, under the stones with the end plugged up and a lot of pin pricks in the hose to let the gas out in small bubbles?
Can you give us a pic of how it looks now Golem? I never worked with CO² with aquarium use..