Green water means there is floating algae in the pond. You may have a filter and UV-C lamp that cannot handle the amount of water. Keep approximately the following: an 18 watt UV-C filter is recommended to keep a pond up to 9000 liters clear. Tip: If you also have fish in the pond; never overfeed. Make sure the fish eat the scattered food within a few minutes. This is because food that is not eaten is a food source for algae.