it is not a good idea to let your carbon stay in the tank for four weeks. carbon only has a life of about 3 weeks and that is pushing it. if your carbon goes longer, it will start to leach back into the tank what it has removed. if you experience a milky cloud and the filter is the culprit, more than likely what you have done is cause the bacteria to increase to not only handle the load of what the fish are producing, but also what the filter is generating back into your tank. when you change your carbon, get a wipeable marker and write the date on the side of the tank in an inconspicous place. that way you are not letting the carbon stay too long after it is dead.
you will need to rinse carbon for about five minutes squinching the bag to move around the carbon and remove any dust that may be trapped between peices.
semper fi