The way to make your water heater last longest, from the tank corrosion issue, is to pull the screwed in
sacrificial anode rod and screw in another one every 5 years or so. Once you pull one, you will be able
to gauge how fast your particular water eats them up. They corrode preferentially to protect the tank.
All "glass coatings" have some 'holidays', or pinhole where they didn't cover. This is where the tank
corrodes thru. As long as you still have some anode left, the tank won't corrode significantly. As soon
as the last of the rod is gone.... off to the corrosion races. Pretty easy to do, look for a 1 or 1 1/4" diam
hex head, often brass, screwed into the top of the tank. Shut off water, drain the lines down a bit, then unscrew
it (sometimes it takes a good bit of torque) and see how it is. It will likely be mostly or totally gone if
the heater is over 5 years old.
$15 to $25 will get you a new one, good for another 5 years on the average.
You ever wonder what a more expensive "long life" water heater has different? A Stainless tank? Thicker
glass lining? Nope. Two anodes from the factory.
Bill