Actually, a natural pool is one that is built similarly to a traditional pool but is cleaned and filtered with aquatic plants. These types of pools have been built extensively in Europe and are increasing in popularity around the globe - even for public use. They can be in the shape of a traditional pool or like a natural pond. The plants compete for nutrients with algae and the plants, if done right, win - so no green scum. They are aerated with a simple filter, and a skimmer takes junk off the surface. There are many resources - here is one if you are interested: http://www.inspiration