Natural Swimming Pool/Ponds (NSP's) are a chemical-free, low-maintenance alternative to traditional chlorine pools. The water is clean, crystal-clear, and filtered naturally using a built-in biological filter.
New Natural swimming pool movement has produced some awesome natural pools with plants, fish, everything! See http://www.interiorholic.com/outdoors/decks-patios-porches-pools/natural-swimming-pools/ for more info on this pic. ~ebm