Once the fish have been in the water for a little while and dirtied it up, the water is pumped up and out of the fish tank and into the hydroponic growing medium that is holding the plants, where it flows and filters through the roots of the plants. With the help of billions of bacteria in the growing medium, nitrogen is broken down into nitrites and nitrates which the plants then absorb.
The longer the water flows through the roots of the plants, the more the bacteria breaks down the nitrogen, thus the cleaner the water gets. Once the water has reached the end of the growing system, either through the use of gravity or various pumps, it then flows back into the fish tank as virtually clean filtered water.