Why does this happen and how do I prevent it
Cloudy water is caused by tiny particles of waste suspended in your pond water. Whilst cloudy water is unlikely to affect the wellbeing of your fish or plants, it is an eye sore and can affect enjoyment of your pond.
The primary way to resolve cloudy water is to remove excess waste and establish a regular maintenance routine. Regularly treating your pond with a sludge treatment will help to keep waste levels down.
If you keep fish in your pond, it is worth assessing your feeding and stocking levels, as over doing it in either of these areas will lead to larger levels of waste.
If cloudy water is becoming a frustrating and common problem in your pond you could consider using a pond filter which will help to remove excess nutrients and waste products like sludge. A Filter will also create a healthy biological filter in your pond to reduce water quality issues and create a better environment for fish.