Why do fish tanks turn green and how can this issue be prevented or resolved?

Fish tanks turn green due to an overgrowth of algae, which is caused by excess nutrients and sunlight. To prevent or resolve this issue, you can reduce the amount of light the tank receives, limit the nutrients in the water by not overfeeding the fish, regularly clean the tank and filter, and consider adding algae-eating fish or plants to help control the algae growth.