If the water in your area is particularly hard, then it is a good idea
to install a second anode rod in your water heater. If your water heater has a hex-head screw on top of the water heater, then you have one anode rod in your tank, and you can easily install a second by using a combination anode rod. (If there is no hex-head screw on top of the water heater, then your anode rod is either hidden and you can still install a second combination anode rod, or the anode rod installed in your water heater is a combination anode rod which means you cannot install a second anode rod.)
Combination anode rods are attached underneath the hot water outlet. In order to install a combination anode rod, you will need to remove the hot water pipe nipple and replace it with the combination anode (see Anode Rods). The replacement combination anode rod’s nipple must be longer than the insulation on top of the water heater—anywhere from 2 for 6 inches.