It is worth being precise about what this claim is and is not. It is a description of how a planted filtration system can keep pond water visually and biologically balanced, which is a design and ecological outcome. It is not a medical or health claim about water safety, and no such claim is made or implied here. The ecological principle is well established; how it is applied, the plant selection, the ratio of planted zone to swimming zone, the way water moves between them, is where the design work actually happens. To see that ecological approach applied to a real swim pond, see ecosystem pond design Leicester.