The Hagen System works fine but is just DIY CO2
in a pretty package. As Kristin pointed out it's over rated as far as the Aquarium size that it is good for, so they are obviously targeting a much lower level of CO2
than generally recommended.
The article that rkilling linked is the one I used when building my DIY CO2
(way back when before going pressurized) and is very informative. I used this GWAPA article
to build my inline reactor (with a few minor modifications to reduce size and account for parts available at my hardware store). Champagne Yeast is really good and can extend the life of the mix to about 4 weeks if you can track it down. The bread yeast works just fine, but you have to change the mix more frequently.