You will find a complete set of instructions included with your Eheim CO2 System. You should read them completely and follow them exactly. Here is some information that will prove useful when purchasing plants. I have put it in a question and answer format for ease of use. The Biological Filter Introducing fish to a new aquarium is make or break for most beginners.
Beginners tend to lose a lot of fish during the first few weeks or months of a newly setup aquarium.
The biggest reason that this happens is because the new aquarist is unaware of what a nitrogen cycle is or how it works. The days of the old under-gravel filter may be coming to an end. At least this help sheet assumes that you did or will. You can put any filter media into the Eheim that suits your needs, but few will surpass the quality of the Eheim media.