I thought about effective CO2 mixing for a long time. I've tested almost everything: diffuser made from wood, air stone, glass diffuser, I've tested CO2 injected on canister filter inlet, etc.
First one is ok but you have to replace wooden airstone every ~2-3 months. Second and third solution is not effective at all. The last method is ok but some of the gas bubbles still going trough the filter and being wasted, bubbles which was not dissolved inside the filter flow out trough outlet pipe and gone, anyway at least is not elegant way.
I decided to make my own mixer from water filter housing
Basically it's not my idea, and you can find plenty of similar solutions on Internet, but I'll try to describe how mine is made.