There's one set up in our store here too, Shane. The plants aren't much bigger and there is more hair algae in the one with the CO2 thingy. Maybe they just aren't maintaining it properly.
Dippy, a DIY (Do It Yourself) system consists of a 2 litre pop bottle, some air line, sugar, yeast and water. Here's some good reading.
http://www.thekrib.com/Plants/CO2/
It's a little tricky to get the right mixture and know when to change it, but once you get the hang of it, it's very easy to maintain.
I use two cups of sugar and 1/2 teaspoon of yeast and change it every two weeks for a 40 gallon tank.
Monitor the Ph and Kh to gauge the CO2 concentration. There's a chart on that page I linked to.
If you have anymore questions, please ask.