Both the Bio-balls and the Sintered Glass Bio-Cylinders are for bacteria to grow on for de-nitrification. You will find many different options about the effectiveness of both products and the entire idea of having them in your filter. I've have tried both ways with mixed results.
micha118833;736201 wrote: I'm not a big fan of the bio balls, I like using LR instead. Seemed to help out on Nitrates on down the road, at least for me