It really depends on how much work you want to do do. Since you have an AC filter, chuck's reactor will work great. Also, what is your CO2 PPM?? If it aint broke - why fix it. I plan on injecting CO2 through my AC 20 for my 5.5 (when I get there). Since it is such a small tank - I am not worried to much about effeciency. Plus I want as little mechanics in my 5.5 so I can see only aquascaping.
I built an external reactor for my 30G for about $15 and 1 hour of time, but I have a canister filter. This same reactor can be had for about $60 online. This will show you the $$$$ saved by DIY.
Further, there are many "retail" reactors available but they all do the same thing as Chucks reactor - so I say build your own and you can be assured of the quality (or for me - lack of quality :laugh: ).
Depending on the size of the tank - I would stay put myself.
Good Luck
Aries