Gravity filters, also known as box filters, must be placed at the highest point in the pond system (above the pond water line or at the top of a water fall whichever is higher). Gravity filters come with or without integrated UV Clarifiers. Recommended pond sizes below are based on typical ponds with average sun exposure and fish stock in the Perth region. Heavily stocked ponds in full sun positions may require greater filtration. Please see our fact sheets to help you choose the right filter for your pond or if you need extra help ring us for advice.