Muriatic Acid (hydrochloric acid 31.25%) will dissolve your gas tank. Other than that I can not think of any reason why you wouldn't want to use it.
Unless your tank is made of plastic or hastelloy C276, I would not use muriatic to clean it.
Muriatic is good for dissolving scale from rust and calcium deposits on THICK steel and non critical parts as long as you know what you are doing. Putting it into a fuel tank and sloshing it around is not one of the applications that you'll find on the recommended list of things to do with it.
Use muriatic for cleaning concrete before painting/epoxy application, and for descaling your power plant boiler, and cleaning the brick around your fireplace, but keep it as far away from your fuel tank as you can.
