Natural filtration swimming pools. These things are wicked. They use water-based plants as natural filters, and ignoring the need to maintain the very minimal pump to aerate/circulate the water, the bloody things self-sustaining. PLUS, if you do it right (which is silly easy), the water is crystal clear and so incredibly pure.
There are companies, like the one linked to here, that provide services for brand new pool installs, as well as conversion from traditional chemical pools.
They are the bomb dot com.
I want this