that fish store lied to you twice...
The co2 yeast reactor probably won't raise the CO2 noticably in a 50G, AND who in the world told you those plants were demanding?
They don't need CO2 at all. They ARE fast growing, but they don't need extra CO2. Generally, plants that require CO2 are the super short ones and the bright red species. Besides, if you have standard lighting, the low-tech method will work fine for you.
Lots of people are under the impression that plants need extra CO2, but they evidently they forget that bacteria and fish produce CO2 through respiration (breathing).
You can probably get a more thorough answer in the plant forum. :)