Fish tank water usually turns green due to an excess of algae in the tank. A sudden algae bloom can be caused by overfeeding the fish, overstocking or direct sunlight, among other causes.
Most fish do not need to eat multiple times a day or even every day. Find out the feeding requirements of the fish in the tank. If there are food particles floating in the tank even 10 minutes after a feeding session, there could be overfeeding. The excess food provides nutrition to the algae in the water, causing it to multiply and turn the water a greenish color. Cut back on the food till all the food that is given is consumed soon after the feeding session.
Another cause for excess algae in the tank is dirty water. Algae grow rapidly when the nitrate levels in the water increase. To prevent this and to keep the tank clean, change 25 to 30 percent of the water on a frequent basis. Algae growth also accelerates with too much light. Move the aquarium away from direct sunlight, or keep the blinds or curtains drawn. If the aquarium has its own artificial light, limit the amount of time that it is kept switched on.
Too many fish in a tank can also cause the water to turn green. Overpopulation causes an increased bio-load on the water, with the existing bacteria trying to keep up with the large amounts of ammonia produced by the fish.