Cloudy water is caused by three main factors and identifying which one is to blame is the first stage in fixing the problem.
Most seriously, nitrite will cause cloudiness and is toxic to fish. Water testing will reveal this and the answer is to review your filtration. In new systems, a lack of the right bacteria will be the issue and this can be remedied by adding a product such as Aquarium Munster's Bactosprint to introduce organisms that will get to work improving water conditions.
Ironically, the wrong type of bacteria can cause cloudiness and as they can pass through most filters, they can be persistent. As above, adding the right kind of bacteria to compete can prove effective and other products exist to get your system back in balance. Algae can cause green cloudiness and the ideal answer to this kind of issue is an ultra-violet clarifier which can kill and clump together individual cells as they pass through the unit.
The last main cause of cloudiness is particulate matter such as the dust found in new gravel or debris disturbed by fish rooting in the gravel. Products can be added to the tank to make these particles large enough to be removed by your filtration, but it's often a sign that your aquarium needs a good clean or the filter needs reviewing.