These two NEW style biological filters (read below) from Tetra work under a water pressure system that allows the filter to be used at any point next to the water garden pool. They do not have to be above the water level for operation. For example, as shown in the drawing, it can be buried and used to pump water up to a water garden feature such as a waterfall. It has a special valve on top of the filter that allows cleaning the filter without opening the lid. Also, the filter can be purchased with the optional built in UV Clarifier to reduce algae in the pool.