However, I still think you should consider buying it and here is why:
1. You get a Hagen Ladder with it, and you probably don't have another means of diffusing CO2 into your water column.
2. You can use Hagen canister as a condensate trap for a 3L bottle based DIY CO2 system. This is nice, because it comes with a bracket so you can hang it right on the back of your tank.
You could use 2 x 3L bottles with yeast mix (2.5 cups sugar, 1/2 tsp yeast, fill with tank water to the shoulders of each bottle). Drill holes in your caps, 1/16" smaller than the tubing outer diameter and pull the tubing through (it will be air tight due to friction).
Plumb the 2 3L bottles together with a T valve and run the tubing to the Hagen CO2 can (you'll need to drill an additional hole in that too). Run the exit tube from the Hagen can to your Hagen Ladder in the tank.
It will take about a day for the CO2 to get up to snuff, so I would suggest getting 4, 3L bottles, and setting up 2 the day before you need to make a switch.