Why does fish tank water turn cloudy after a weeks change?

Fish tank water can turn cloudy after a water change due to several reasons, primarily the disruption of beneficial bacteria during the process. When you change a significant amount of water, it can disturb these bacteria, leading to a temporary increase in particulate matter and algae growth. Additionally, organic waste and uneaten food can contribute to cloudiness as they break down. It's typically a sign that the tank is adjusting, and it should clear up as the biological filtration stabilizes.