A green koi pond is usually caused by an overgrowth of algae, tiny plant-like organisms that thrive in water with excess nutrients and sunlight. When there is too much sunlight, especially on warm days, and high levels of nutrients like nitrates and phosphates from fish waste or decaying plants, algae multiply rapidly, turning the water green and murky.
While some algae are natural and even beneficial in small amounts, too much can reduce oxygen levels, stress koi, and make the pond look unhealthy. To reduce green water, it helps to limit sunlight exposure by adding shade with plants or structures and controlling nutrient levels through regular water changes and proper feeding. Installing a good filtration system and using UV clarifiers can also help control algae growth by removing suspended algae cells. Avoid overfeeding your koi, as uneaten food adds nutrients to the water.
