Why Does Aquarium Water Get Cloudy? Cloudy aquarium water can be a puzzler, right? One moment it's crystal clear, the next it's a murky mess. This can happen for a few reasons. Maybe you've just cleaned the tank, or perhaps you've added chemicals too quickly to your marine aquarium, causing a snow-like precipitation. Another possibility is that a food or chemical has dissolved in the water, leading to the cloudiness. Remember, these are the three most common causes of cloudy aquarium water. So,