Before the filter has time to develop its own population of micro-organisms, it is not uncommon for them to grow in the water itself. This ‘cloud’ of microscopic life can give the water a hazy or cloudy appearance. In itself it is not harmful, and should pass within a week or so (a partial water change can be done if it is very severe). However, you should check the water for ammonia and nitrite to ensure they are not also high. If they are, add Tetra SafeStart to the water.