The only natural water purifiers and water filters in ponds are plants. Aquatic plants absorb nutrients and dissolved substances. When living they pruify the water when dead and decomposing they 'pollute' the water. If you are looking for natural mechinal filtration of pond and lakes it is forms of zooplankton mostly belonging to the Crustacean group named Cladocera. Also freshwater mussels for their size filter lots of water. Zebra mussels filter lots of water but they are considered exotic species and invasive and cause some significant big problems in pond / lake systems.
https://mde.maryland.gov/programs/water/WetlandsandWaterways/Documents/August%201,%202023%20Supplemental%20Briefing%20from%20Waterkeepers%20Chesapeake%20and%20Lower%20Susquehanna%20Riverkeepers/1.w%20MDE%20Draft%20Mussels%20Study.pdf
Mussels are well-known for their ability to filter water. Mussels are filter-feeding organisms that remove particulate nutrients (seston) from the water column. Particle filtration by mussels range from 0.50 to 3.7 Liters per hour per mussel. The Eastern Elliptio is estimated to filter 16.5 gallons of water per mussel per day (24 hours). Through daily feeding activities, mussels improve water quality by reducing nutrient transport in riverine systems. Nutrients taken in by mussels are either stored in the tissue of the mussel (soft tissues and shell material), translocated from the water column to the substrate in the form of biodeposits (feces and pseudofeces), or returned to the water column as excreted (soluble) nutrients.
Assimilation of nutrients in mussel tissues Mussel tissue is composed of 8-13% Nitrogen (N) and 1-4% Phosphorus (P). Mussel shell is composed of 1-2% N and less than 1% P. A single mussel is estimated to contain approximately 0.7 g N and 0.009 g P.
Video example of filtering ability
https://www.instagram.com/minnesotadnr/reel/C3z8WjgPwEe/Harnessing Mussels to Filter Fresh Water
https://www.greatlakesnow.org/2024/08/harnessing-mussels-to-filter-fresh-water/
