The most likely reason for the water to be cloudy in a coldwater aquarium is lack of, or insufficient filtration to cope with the waste produced by the fish. Cold water fish are much larger than their tropical counterparts, so therefore need a more efficient filtration system. The most immediate way to clear the water is to carry out a partial water change of say 25% and reduce the amount of food given. The addition of Interpet filter aid will also clear the water, but only in filtered aquariums. While this will clear the water it will not offer a permanent solution. It would be wise to look into the cause. New aquariums also may suffer from cloudy water despite sufficient filtration. Read about new tank syndrome.