I have mine under my tank in a cabinet. Ran the airline up and into the tank and directly into the intake for my canister filter (just stuck it in the grill and the bubbles go right up the tube (you can see and hear them... that is how I know it is working, lol).
It really does make a difference running it through your filter. No bubbles at all so I have to assume that I am getting the maximum about of co2 I could with the DIY with my recipe (i.e. I am not gassing out any co2). Now I wish I had more light to help maximize teh benefit of the co2.