How to keep the water in cement fish pond green

The water is green, because it produces green algae. I have a cement fish pond with 4 cubic meters of water, which has been flooded with the strong sunshine. At first, carbon fiber, non-woven fabrics, coral stones and sedimentation tanks were filtered with water pumps, and the effect was not obvious, but more and more. Later, copper sulfate solution was added to the sedimentation tank (to prevent it from being poisoned by fish food), and it just started to have an effect, once a week on average. I bought a fishing medicine to improve the water quality, which seems to be called granular oxygen. It is put once a week, diluted and sprinkled on the water surface of the pond, and then used alternately with copper sulfate. The water quality is very good and crystal clear, and even the breeding small fish can be seen clearly. It is best to use this kind of thing in sunny morning, and I usually choose more than 7 o'clock in the morning.