Bubble filters work in a similar way to under gravel filters. The rising air bubbles (provided by an air pump sold separately) draw up the water flow through the filter. The water then returns to the aquarium. Bubble filters are great biological filters, which means that good bacteria live in them and as the water passes by the bacteria feed on the fish waste and clean the water. Bubble filters are old school - used most before mechanical filters became popular. Traditional fishkeepers still prefer them as they cause very little disruption to the aquarium - keeping the water still. They also oxygenate the water and are very cheap to implement. You can run several bubble filters from a single powerful air pump.