How to get rid of green water
If your aquarium water looks like pea soup, you probably have green water, which is caused by a proliferation of free-floating, single-celled phytoplankton. Unfortunately, they replicate so quickly that you cannot flush them out with large water changes. Green water can come from too much lighting (especially if the tank gets direct sunlight sometime during the day), an excess of nutrients (such as accidentally double-dosing fertilizers), or an ammonia spike (such as from a new tank that has not been cycled yet or overfeeding by a pet sitter). To get rid of green water, you can blackout the tank for at least a week, which is hard on your plants. Another option is to purchase a UV sterilizer, which will kill off the algae within two to three days.
For more information, read our full article on how to fight algae in your fish tank.