When I was first starting with CO2, I set up my tank with *just* the
CO2 and the filter - CO2 flowed into the Eheim cannister that flowed
out into 2 biowheels (which are a favorite filter of mine.)
I also rigged the tank with a Dupla Continuous-indicator CO2 test-kit,
but other than that, nothing was in the tank (i.e. no gravel, no fish,
I found that with the biowheels in, no matter how much CO2 I added, the
test-kit never showed any more disolved CO2 than when I turned the CO2 off.
Then I removed the biowheels from the system and lo, the test-kit showed
CO2 in the water. I had the tank filled so that there was no splashing,
and the lid of the biowheel was closed, so I'm afraid there's no way for
me to draw any other conclusion than that biowheels are inherently
incompatible with CO2 injection.
In Austin TX, where it ain't just playin' at bein' hot...