How to keep the water in cement fish ponds green

Water is green because it produces green algae. I have a cement fish pond with 40 cubic meters of water, and the green algae have been flooding under the strong sunlight. At first, carbon fiber, non-woven fabric, coral stone, sedimentation tank and water pump were used for filtration, but the effect was not obvious and more and more. Later, copper sulfate solution was added to the sedimentation tank (to prevent it from being poisoned by fish food after directly adding pond water), and it was effective at first, once a week on average, with a ratio of 0. I bought a kind of fishing medicine to improve water quality, which seems to be called granular oxygen. Put it once a week, dilute it and sprinkle it on the surface of the pond, and then use it alternately with copper sulfate. The water quality is very good and crystal clear, even the farmed small fish can see it clearly. This kind of thing is best used in sunny mornings. I usually choose after 7: 00 in the morning.