Do a 50% water change. Run the water in a separate container till the water is the same temperature (hopefully you have something to hold 20 gallons of water in!) and then take out half the water in the tank. Replace it with the new water, as that should help the cloudiness. If you replace all the water in the tank, the fish could go into shock because water has to go through a whole new bacteria cycle when it gets into the tank, and then the fishes' systems could get messed up too badly. A 50% water change won't do harm, and it'll keep the water clean for the fish.