Why is my fish tank green after a water change?

The green color in your fish tank after a water change is likely due to an overgrowth of algae. Algae thrive in water with excess nutrients and sunlight, which can occur when the water is changed too infrequently or when the tank is exposed to too much light. To address this issue, you can try reducing the amount of light the tank receives, cleaning the tank more regularly, and possibly adding algae-eating fish or plants to help control the algae growth.