Time Has Come, Said The Walrus, To Speak Of Many Things: Of CO2 Reactors...
Hi,
I believe running that diffuser in some kind of reactor, even the most basic would serve you better.
While bubble rates are poor estimators of CO2 use, it sounds as though you are getting plenty of gas into the tank, you are failing to get the gas into solution and circulated around the tank.
If bubbles from the diffuser are visible to the naked eye hitting the surface, halving the amount of the water column is going to reduce the total amount of CO2 in solution, significantly.
Then add the problem Jonny mentioned.
As far as I can tell most diffusers are designed to operate most efficiently in 10 feet (3 meters) or more of water in about 3 miles per hour (4.8 kilometers per hour) of flow.
In my ever-humble potted plant opinion, any type of CO2 counter flow device is a major improvement.
Biollante