Just an FYI on BioWheels... The thing I like about them is that I can clean the heck out of the cannister filter's filter material without worrying about doing too much damage to my beneficial bacteria colony - since most of it lives on the biowheels, which never get cleaned.
I've got 2 Biowheel 60 pros (i.e. 4 full-sized biowheels)on my planted/pressureized CO2 injected 135g tank. My CO2 usage is not noticeably higher since going with the biowheels.
However, I think this could be the reason:
The Biowheel 60 pros run in completely enclosed compartments with little hinged doors that you can raise up to inspect/maintain the biowheels. My theory (and it is ONLY a theory) is that since the compartments the biowheels run in are enclosed (not airtight but mostly enclosed), the CO2 level in the air inside the compartments rises to approximate the CO2 level in the water running over the wheels... so no large loss of CO2 as the water falls over the biowheels since the concentration of CO2 in the water is nearly the same as the concentration of CO2 in the surrounding air (inside each biowheel's compartment). Now I could be completely wrong. I *can* say, however, that I run about 30 ppm CO2 supplied by a 20 lb cylinder for a 135g tank with 4 biowheels (2 60 pros) and I change the CO2 bottle about every 10 months.
The smaller HOB filters with biowheels DO expose the wheel to the open air. You can see about 1/4 of the wheel rotating out in the open when you look at the filter. With that setup, I could easily imagine a large CO2 loss. I guess the bad news is I don't think they make a smaller cousin to the 60 Pro with a similar "fully enclosed" design. A 60 Pro would be too big for smaller aquariums.
In summary: My answer to the "Will a biowheel cause CO2 loss?" question is "Yes" and "No, not really."