Originally Posted by Alex09
Undergravel filters are crap. You will have to tear up your whole tank just to clean them.
biOrbs do not have undergravel filtration. Basically, at the bottom of the aquarium, there are a collection of ceramic stones which are used as bio-media. These sit below the gravel and water is pulled through these and onto the second stage of filtration, which consists of a small sponge cartridge through which water is pulled by the bubble lift. This explains the bubbles going up the middle. The clean water returns through the bubbles.