I actually ordered a spare anode rod for future replacement in my new 30 gallon electric hot water tank. (I had replaced one before in the previous tank. )

But I never know what material is the best to buy. Magnesium or Aluminum? What is the difference? Also, when I cut the new rod...should it be 4 to 6 inches from the tank bottom?

I have ordered aluminum, only because it was cheaper and I like cheap 6 year tanks, too as my water is public water system but it seems to eat the rods fast. I believe it is hard water, a lot of white deposits collect in hot water tanks/ showerheads...etc.

Thank you.