[slodave]

Bacterial Blossom - Often cloudy water doesn't appear the instant an aquarium is set up. Instead it appears days, weeks, or even months later. In these cases the cause is usually due to bacterial bloom. As the new aquarium goes through the initial break in cycle, it is not unusual for the water to become cloudy, or at least a little hazy. It will take several weeks to several months to establish bacterial colonies that are able to clear wastes from the water. Over time that cloudiness will resolve itself.

Decaying plants or excess food that remains uneaten can also cause the milky water seen in bacterial bloom. Regardless of the cause, don't panic over bacterial blooms. Keeping the aquarium very clean by removing debris such as decaying plants and uneaten food, vacuuming the gravel regularly, and performing partial water changes, will quickly resolve most cases of bacterial bloom. Cut back feeding to every second or third day, which will cut down on excess food decay.

Taken from about.com...