Satoyama II: Japan's Secret Watergarden (2004)

Each home has a built in pool or water tank that lies partly inside, partly outside its’ walls… A continuous stream of spring water is piped right into a basin, so freshwater is always available. People rinse out pots in the tank and clean their freshly picked vegetables. If they simply pour the food scraps back in the water, they risk polluting the whole village supply. However, carp can scour out even the greasy or burnt pans. They do the washing up in Satoyama villages. This traditional arrangement is called the riverside method. It’s used all over Japan. Cleaned up by the carp, the tank water eventually rejoins the channel.

Unfortunately the movie Satoyama II: Japan's Secret Watergarden is not yet available on HBO Max.