A 2" x 12" should work for you but need to be at return back to the tank not at the intake , and it flows in at the top of the reactor and out the bottom to the tank. I've use a 2x12 with a mag 5 but I added mini bio balls to break up the flow and co2 and it always stayed full for my 55g tank only draw back you lose a little bit of flow with the bio balls inside. And like Herns said you might need co2 hose line .
It's only a 20 gallon with a fluval 206. I am going to be setting up a 90 gallon I got in the next couple of months. With the set up I have now when I turn on the pump I have a hard time getting the reactor to fill up with water even with a bleeder line at the top. The reactor is directly above the filter intake. With a 2" or 3" diameter will it be harder to fill? Is there an way around this?