The bio-filter plates provide lots of surface area for beneficial bacteria to colonize. As the water trickles over the plates it is exposed to two types of bacteria living on the plates. A small portion of the ammonia is also lost as a gas to the atmosphere surrounding the plates. The trickling action oxygenates the water and off-gases CO 2 as it passes over the plates much like a waterfall oxygenates water in a stream. The water leaving the bio-filter and the ozone contactor is then collected in a large tank called the sump.
Pumps draw the water up from the sump compartment and back to the respective tanks.
The water may be pumped directly into the exhibit at midwater level or into the bottom of each tank through the undergravel filter. This action prevents the buildup of organic particles and debris in the gravel bed. The principals and procedures of filtration are the same for the Aquarium's smaller freshwater and saltwater exhibits. In many tanks at the Aquarium a simple undergravel filter is used to clean the water. A filter plate is placed in the bottom of the tank and covered with a layer of gravel.