DIY CO2 and HOB filters?
 

I'm using whisper filters on tanks with DIY CO2. As long as the water level in
the tanks is kept high there's not that much turbulance. I used to run the CO2
air hose into the filter intake, with very good results, but I now use a really
cheap ($4 - $6) diffuser, the Rena Micro Bubbler from www.petsolutions.com.
This is even more effectiveand lots quieter. However, the diffuser tends to
leak where the end caps attach to the media tube, but a little silicon glue
fixes that.