DIY Co2 Instructions for a Freshwater Aquarium Planted Tank
The height from the bottle to the aquarium isn't an issue; it's the
depth that the airline goes into the water that causes back-pressure.
I have two non-return valves AND an airstone AND the bottle 3 ft below
the aquarium, and the pressure is strong enough to produce a good
stream of bubbles. So use non-return valves, and airstone if you want.
I could never get my CO2 to work through an airstone that was placed at
the bottom of a 55 gallon tank. Heck, I had a hard time blowing into
the airline and pushing out bubbles with my own lungs. What I was able
to do was using an airine connector at the end of the airline tube and
inserted the exposed nippled into the plastic basket (the guard on the
suction portion of the filter) on my canister filter but this could
easily be done also on an external filter such as an Emperor 400.