Wetland filtration for outdoor ponds Constructed wetlands have grown in popularity over the last couple of years due to their natural aesthetic, ease of care, and impressive filtration properties. Wetland filters are a great option if you want to keep a lot of fish or plan to swim in your pond. A wetland filter works like a biological filter, which creates a waterfall cascading into your pond. The wetland, however, creates an area in the pond thick with naturally-filtering plants, as well as rocks and gravel. In addition, a wetland is more effective at filtration, which is why more pond owners are incorporating them into new or existing ponds. Wetlands are designed to work like Mother Nature and are often considered to be the kidneys of the earth. Just like kidneys filter toxins out of the human body, wetlands filter toxins out of water. A wetland filter slows the velocity of water by 93 percent, and in a natural system like this, low pressure is best.