Green water in a freshwater aquarium can be a common issue for aquarium hobbyists, but it can also be easily prevented or treated.

Green water is caused by microscopic algae that proliferate in the water, giving it a green tint. This is often a result of high levels of light and nutrients in the aquarium. While it may look unappealing, green water is not necessarily harmful to fish or other aquatic creatures in the tank.

To prevent green water, it's important to limit the amount of light that enters the aquarium and to keep the water clean and well-filtered. Regular water changes and reducing the amount of food given to the fish can also help to keep algae growth under control. 

If green water has already become a problem, there are a few steps you can take to clear it up. One option is to use an algaecide, which is a chemical treatment specifically designed to kill algae. However, it's important to be careful when using algaecides, as they can also harm other aquatic creatures in the tank.

Another option is to use a UV sterilizer, which will kill the algae as it passes through the unit. This is a more effective and safer solution, but it can be expensive.

Running a blackout can also be an efficient way to remove green water.  Simply do a 30% water change, add an airstone connected to an air pump, and put a blanket or towel over the aquarium so no light can get it.  Turn your aquarium light off and leave it dark for three days, no peeking and no feeding. After 3 days, remove the cover and do a water change. 

Finally, you can also simply wait it out. Green water will naturally clear up on its own as the algae die off and are filtered out of the water. This may take several weeks, however, so be patient.

In conclusion, green water in a freshwater aquarium can be easily prevented or treated. With the proper maintenance and care, your aquarium can remain clear and beautiful for you and your aquatic creatures to enjoy.