Try feeding your CO2 directly into the canister intake, this way you can increase surface agitation by repositioning filter return, this will cause more CO2 loss. You can also reduce the efficiency of your diffusion method, try a glass diffuser for example. Otherwise play with the mixture as you suggest, personally I'd increase gas exchange with more surface agitation.
Also take a look here for some ideas: http://www.gpodio.com/diy_co2.asp
Hope that helps