The water in an aquarium will not turn green on its own without the presence of algae. The green color in an aquarium is caused by single-celled algae that float freely. These algae multiply quickly under certain conditions, creating billions of cells. When enough cells are present in the water, a green tinge will be present throughout. While this is not dangerous to the fish, it is ugly and annoying.
Turn Out the Lights
Direct sunlight beaming in from a window is the most common cause of algae growth in aquariums. They don't need to be lit up all the time, as light just causes green algae to grow faster. Minimize the hours the tank spends in direct sunlight by closing the window blinds or curtains for part of the day. If the tank has its own lamp, turn it off during the daylight hours and just leave it on at night.
Change the Water
Once the lighting has been changed, suctioning out the gravel will help remove a lot of the green algae that is present. Removing about 25 percent of the old water and replacing it with fresh will reduce the concentration of green algae floating around in the tank. Replace this amount several times until the water is clear and remains that way. Changing the filter at least once a week to help maintain clear water helps a lot.
If nothing works to eliminate the green tint from the water, the next step is the more expensive but thorough UV sterilizer unit, whereby aquarium water is pumped through a tube and exposed to UV light, killing all of the single-celled algae. Such a unit is useful for destroying disease-causing organisms like parasites that live in fish. The UV sterilizer kills the algae but won't remove the dead cells from the water.
Vacuum Out the Dead Algae
Thanks to decomposition, once the algae are dead, their continued presence in the tank is enough to cause another algae bloom. Removing it as quickly as possible by suctioning the bottom of the tank is the best way to prevent algae from returning. Vacuuming out the bottom of the tank, removing as much dead algae and fish waste as possible, will create a more healthy environment. Afterward, add fresh water to refill the aquarium.
Another Cause: Blending Colors
Assuming you know your aquarium is algae-free, but green water remains, something green may be reflecting into the water. Some light bulbs will cast a green tone that is visible only in the water. Change the color of the light bulb in the tank cap. If the bulb is yellow and the background design is blue, a green tone could result, making the water appear green when it is actually clear. Try using a white light bulb instead to stop the blending of colors.