River Flow Biological filter's allow more air to enter the water, making these gravity filters ideal for larger fish, such as Koi and Goldfish. These filters need to be above the water level, in order for the water passing through the biological filter to gravity feed back into the pond. River Flow's simple design makes cleaning the pond a quick five minute job. All that is needed, is to pop off the lid and remove the sponges and give them a rinse off with the garden hose. The Bio Balls located in the bottom of the filters shouldn't be cleaned, this is where the good bacteria colonizes. We recommend the use of Clarifier Bacteria for when first starting up your filter or whenever you are giving the pond a good clean out to restore any bacteria lost.