To clean cloudy water in your aquarium, use a mechanical filtration system and a protein skimmer. Common causes of cloudy water include excess nitrates, phosphates and DOCs, so eliminating them should clear the water. Treatments may vary depending on the cause.
If the cloudy water is green, it is due to algae growth. Possible causes of algae growth include too much sunlight, excess nutrients, phosphates and nitrates. Consider moving the tank out of direct sunlight if necessary. Test the water for excess phosphates or nitrates, and treat the water as needed. Phosphates may require a phosphate remover, and excess nitrates requires a water change. It is also important to keep the filters clean.
If the cloudy water is white or grayish, the gravel in the tank may need to be washed. Drain the tank, wash the gravel thoroughly, and then refill it. If this does not solve the problem, check the pH balance. If it is high, add the appropriate conditioners. Using reverse osmosis water or a unit that makes reverse osmosis water may clear up the water.
Sometimes cloudy water is caused by bacterial bloom. This usually clears up naturally within a few weeks. Removing uneaten food and decaying plants may help to clear the water faster. It may also help to vacuum the gravel and do partial water changes on a regular basis. Flocculates, or water clarifiers as they are often called, help to clump debris together, making it easier to remove.