Why is my tropical fish tank water cloudy?

Aquarium water can turn cloudy due to bacterial growth, minerals leaching into the water, or presence of microscopic debris that the filter cannot remove. A bloom of bacterial growth is the most common cause, particularly in newer tanks. It can take months for a new tank's ecosystem to establish itself.

Even in a more established tank, anything that upsets the chemistry of the tank can cause water to turn cloudy. A dead fish, overfeeding or simple fluctuations in water chemistry can result in rapid growth of bacteria that makes the water cloudy. Over-cleaning filters and making considerable changes to the water can also upset the water chemistry and turn the water cloudy. Cutting back on feeding, vacuuming the gravel and making a small partial water change often clears up the tank within a few days.