Panda,
I don't think a reducing Tee would work very well for your application, but then again I have never personally used that type of a setup either. The flow of the output of the FX5 is just so fast that I can't see the bubbles being able to dissolve much at all before being shot out the return, even if the CO2 was bubbling through an airstone inside the Tee. Maybe someone else that has some actual experience with that setup could comment more on it. A venturi/mazzei would work, and it would work well, but a venturi requires a water pressure drop across the venturi itself in order to pull the CO2 in and break up the bubbles. The FX5 will see that as extra head pressure, so the flow capacity of the canister filter will be cut. I don't know by how much, because I don't know the flow curve or max head capacity of the FX5 pump, but I do own an FX5, and I would guess that it would be around a 100-200gph cut in flow just based on witnessing how much the flow in the filter changes with clean vs. dirty filter elements.
I would think that your best bet would be something close to Rex Grigg's design. Just do a search for "Rex Grigg DIY CO2 reactor" and you will find lots of sites that have pictures and instructions on how to make his particular reactor and others that are only slightly different. They all work pretty well. The nice thing about that type of design is that it does a good job of dissolving the CO2 without adding much flow resistance to the canister filter output line. I would suggest possibly making the reactor out of 3" pipe if possible instead of 2" though since you have a lot higher flow than most people using canister filters do. You want the downward flow of water in the cylinder to be nice and slow. If you can't do it in 3", possibly make the reactor chamber a little longer instead just to allow the CO2 as long as possible to dissolve in the chamber before going into the tank.
If you wanted to use the Quiet one pump in order to add some extra flow to the tank, leave the FX5 as it is and have a second intake and output in the tank just for the CO2 reactor. Then either use the same "Rex Grigg style" reactor, or make a small vortex reactor that hangs on the back of the tank. Vortex reactors are very efficient as well, and can also be made very compact if necessary. Just do a search for "DIY vortex CO2 reactor" and there are lots of examples and even a few videos on YouTube of them in operation.
Have a good one, Jeremy
