Here are the basics:
Magnesium will provide a little better protection for your water heater so if possible you should use a Magnesium rod.
However, if the water in your area is hard (will explain below), you may need to use Aluminum because a Magnesium rod will erode too quickly. That said, for Baltimore City and most large cities, the hardness is on the border of soft and hard (numbers below) so you can use a Magnesium Anode Rod.
Pro Tips:
>By far the hardest part of the job is getting the old rod out. Be careful! Use a good impact wrench!
>Make sure turn off gas (or electric for electric heaters) and turn the unit off then flush through with cold water so it’s no longer hot.
>Break the nut loose on the old Anode rod with the heater FULL of water to provide more stability (just break loose, Don't take out until you've drained the water.
>Don’t use too much plumbers tape (teflon tape) or the anode won’t be conductive with the rest of the metal in the hot water tank, rendering the anode useless and no protection on tank from galvanic corrosion.
>Since you’ll need to drain the tank out to replace the rod, now a good time to flush it. If possible, when you have the rod out, use a garden hose with a jet nozzle and spray down the Anode Rod hole. This will help flush the tank out.
>When you put the rod back in, just snug it. Then, AFTER you fill it with water, torque it to about 8 ft-lbs.
Water hardness:
Total hardness 75 ppm = SOFT; use Magnesium Rod
Total Hardness between 75 and 150 ppm = Moderately Hard, Use Magnesium Rod
Total Hardness > 150ppm = Hard, Use Aluminum Rod
Ex. Baltimore city total hardness is usually between 85 and 110ppm so use a Magnesium Rod