My tanks have not leaked in 26 years, so I persist in servicing them on a regular basis instead of buying new ones.
Our water is very alkaline, the aquifer being in a caliche formation so mine plug up every 2-5 years. I have 8 to take care of right now.
Once they silt up to the lower element, that element blows and its time to swap (I keep spares ready to go at all times).
I swap and we have hot water restored within 30 minutes. (they are all fitted with flex hoses and the lower drain is fitted with a 3/4" short nipple to accommodate a garden hose)
Then I go to work on the silted one at my leisure. I haul it out on the grass after draining the water, pull off the elements, in/out risers, PRV, remove the dip tube, sometimes the thermostats, the Mg rod, the drain valve.
Then with a high tech tool of bent wire I start scooping the mung out of the bottom while a garden hose is running in the upper element hole. Once it is pristine inside, it is time to reassemble.
Cleaned up female threads, new elements, cleaned up PRV, usually new Mg rod(not convinced this is necessary), new heat trap risers, lube all threads with vaseline (using a thread compound means you will taste it for the first 2 months of installation).
Tips.
-Elements must only be put on tight enough to ensure no leaks, then watch for a few days before closing in. Vaseline here too. Removing them; the socket must periodically be ground flat so that you are into new socket engaging the hex. Occasionally I have put it into the truck, driven to the garage and had the air impact wrench loosen them. Many times I have had to remove elements with a chisel and mallet, making ugly grooves in the hex as you work your way around it.
I don't recall not being able to get one out. I do recall cursing, being concerned that I was making it impossible to remove, and taking an hour to get them out.
Sometimes the element explodes and cannot be twisted out so it is forcefully removed with whatever is available to grab it. Then pieces are left in the tank and teased out later. The double folded elements can be miserable to remove pieces larger than the opening.
The lower drain - most tanks come with a nylon or plastic valve assy which is so designed that if you mess with it, it busts so you buy a new tank. And a small orifice so that you think its toast and go buy a new water heater*, or it plugs so you have to mess with it. As soon as I buy a new tank (its rare) I pull this stupid valve out and install a 3/4", 90° full port ball valve on the end of a 3" nipple in the tank. Vaseline, no need to crank any of these crazy tight. Oh, and to get out a busted off nylon valve...I have a trick.
If it is in storage I put in a small amount of bleach and I seal all the openings.
-I can't wait to try my new electric impact wrench on the Mg rod.
*NB there is no such thing as a hot water heater. ***
***EXCEPT I have one. Well, I installed a water heater under a counter where the time for hot water to reach it was excessive. So that small water heater sometimes heats hot water and thus is one of the rare instances where you can say "Hot water heater".