Water plants are the answer to green water, but because of the design of Koi ponds, and the nature of Koi carp, plants are difficult to nurture, so green water is a long-term problem for Koi keepers.

Tiny single-celled algae multiply rapidly on warm bright days to create the green effect and without competition from plants for light or nutrients, the effect stays. Then the murkiness blocks light to any submerged oxygenating plants, which then grow very slowly, so have little effect on the greenness.

The large population of algae depletes dissolved oxygen concentrations (DOC) and produces carbon dioxide, which in turn can make the water more acidic at certain times of the day. This fluctuation in pH in turn affects the chemical composition of the pond water and is certainly bad news for Koi.