The size of your pond will determine the wattage of the UV sterilizer you need. There are UV sterilizers for ponds, and also UV clarifiers. Both will clear up green water, but UV sterilizers will do a lot more in protecting your fish. You can do an online search "best pond UV sterilizers", and the articles will list both UV sterilizers and clarifiers suitable for ponds. Perhaps kendragon can give his opinion, as he has installed US sterilizers in ponds.
kendragon