FYI - the koi or fish that ate the duckweed and big vegetation could have indirectly caused the current "green" problem by eating and reducing large amounts of green growth. This large amount of plant reduction by the fish resulted in lots of nutrients to be recycled and be available. "Nature abhors a vacuum" so something in the form of a new type plant moved in that likes lots of nutrient conditions and was able to grow because the fish will not eat it. Now the new plant can grow unrestricted based on availability of nutrients. IMO the koi did not really fix the problem they just ate the resultant plant growth that was growing from too many available nutrients. It is very similar to blowing your nose when you have a head cold. Blowing the nose reduces the drainage but it does not fix the problem of the cause - viral head cold. The drainage is just a symptom of a head cold, just as excessive plant growth is a symptom of too many unused nutrients.