If you have blanket weed, try Cloverleaf blanket answer. Used it in my koi pond and didn't have a single strand for 5 years. It's not a chemical but an enzyme that eats the algea.
If it's just green water, try either adding a UV (change the bulb every March), using Barley straw as recommended above or reducing the competition for light and food for the algea by adding more plants to use up the excess nitrates and phosphates produced by the fish and more lillies to cover around 2/3 of the water surface.