Green aquarium water can be a frustrating and unsightly problem for aquarium owners. It's caused by the overgrowth of single-celled algae, which thrive in warm, nutrient-rich water that is exposed to light. Fortunately, there are several things you can do to prevent green water in your aquarium:

1. Reduce Nutrient Levels: Single-celled algae thrive on nutrients like nitrates and phosphates, which can build up in the water over time. You can reduce nutrient levels by doing partial water changes, reducing feeding, and not overstocking your aquarium.

2. Control Lighting: Algae need light to grow, so you can reduce the amount of time your aquarium is exposed to light. Limit the amount of time you run your aquarium light each day, and make sure it's not exposed to direct sunlight.

3. Increase Water Flow: Algae growth can be reduced by increasing water flow in the aquarium. This helps prevent the buildup of nutrients in stagnant water and can also help filter out algae cells.

4. Add Live Plants: Live plants compete with algae for nutrients and can help keep the water clear. Consider adding some fast-growing plants to your aquarium to help control algae growth.

5. Use a UV Sterilizer: A UV sterilizer uses ultraviolet light to kill algae cells and prevent them from reproducing. This can be an effective way to control green water, but it's important to make sure the sterilizer is properly sized for your aquarium and that the bulb is replaced regularly.

By following these tips, you can prevent green water and keep your aquarium looking clear and healthy.