The military uses metal tanks, and has for years. So do thousands of farms going back forever.
Steel or aluminum, how to tell: Get a magnet. (Duh!
)
A solution of
ordinary unscented no other additives household bleach, 1:100, should adequately clean and sterilize the innards. Put one cup of bleach into 99 cups of water (which is 6.18 gallons of water, BTW. If you put a cup into 5 gallons for convenience, that'd be stronger, and wouldn't hurt anything, or you could put 1/2 cup into 3 gallons, same-same.), presto. Spray it thoroughly on the interior, leave it there for >10 minutes, then thoroughly rinse it clean with fresh water.
Assuming it doesn't leak, it's now ready for use.
Were it me, I'd plumb it such that fresh water is piped in at the top and drawn out at the bottom, with valves at each end, to seal it off at your choice and convenience. If you can/care to plumb a purification and/or softening filter set up at the incoming end, so much the better. (Really high-speed: If seasonal freezes are an issue where you are, a solar water-heating set up to keep the tank and piping from freezing in cold months, ideally in some small supporting and insulated shed. You don't need it to be at 72 degrees, 33 or better is sufficient, right?)
Worst case, it'd be a 700 gallon cistern to draw from with any food-grade pump, even a hand pump, or a spigot (with the tank set above ground at least as high as above the top of a 7 gallon food pail/military water jug, for ease of drawing, because you're smart!) and while you might need to filter it again for absolute safety, you'd be starting with water a helluva lot cleaner than anything else. For you + one, that's nearly a full year's supply at a survival bare minimum of 1 gal/person/day.
Even at typical household use rates that are 100x that, 700 gallons is a 3 1/2 day cushion for two people during any future water outage, allowing full normal use of bathroom, laundry etc. (Presuming you have a pump that can pressurize your system after you shut it off from any outside supply lines. Otherwise, you're hauling jugs/buckets. A little red wagon, trash dolly, or garden cart is your new friend: 5 gallons is a 41# load, plus the container wt.) Pretty good stuff IMHO.
So hell yes, I'd install and use it in a heartbeat.
As an inhabited shelter?
Hell no!
Confined spaces like truck and railroad tanks kill people regularly.
If you decide to re-purpose it, you'd more likely want to use it as a sealed storage cache, and if you put stuff in or retrieve it, you want to open it and ventilate it with fresh breathable air for 10-20 minutes before entry, and even then, only with someone standing by outside just in case.
(Who could drag your unconscious ass out in an emergency, via a handy rope and winch. 4 minutes w/o air = brain damage, 6 minutes = all the way dead.)
People who go into vaults like that alone may not always come out.
And a 700 gal. tank would likely be too damned small to want to get into for more than a few minutes anyways.
You can turn a metal can/tank into a shelter, but realize you've essentially created a submarine, so you need to be able to bring in, handle, and remove fresh and stale air, and that people exhale water vapor, so humidity, condensation, dampness, and mold all become a problem in such conditions, followed by respiratory infections and other nastiness, not least of which are things like Legionnaire's disease. Submarines are for people with nuclear reactors, large diesel power plants, or other lotsa-power capabilities, plus humidity control, air filtration and cleaning, and UV lights to kill the bugs.
Hope that helps the thought and planning process.
"There are four types of homicide: felonious, accidental, justifiable, and praiseworthy." -Ambrose Bierce, "The Devil's Dictionary"
