This condensation on the outside of the tank is caused by cold water filling the tank and humid air in the bathroom. There are a few of things you can do. You can insulate the inside of your tank. They sell kits for this and they work fairly well. The tank covers you can put on the outside help somewhat as well. You can also get a mixing valve. This is put on your cold-water supply. It will add just enough hot water to the cold to warm it up. When the tank is filled with water at room temperature it won't sweat.