Some internet info:
For cleaning the tank, different acids can be used. One of the most commonly used acid is the muriatic acid, also known as hydrochloric acid. Other than hydrochloric acid, phosphoric acid is also often used for tank cleaning purposes. However, when either of the acid is used, one has to take an immense amount of care while undertaking the cleaning procedure. The other ingredients, which can be used for tank cleaning include vinegar, acetone, detergent, hot water, etc. It is recommended that the cleaning procedure be undertaken in open air as the fumes may be hazardous.
Acetone: You will need about 2 to 3 quarts of acetone for the process. Close all the holes of the tank using drywall screws. Pour about 1 quart of acetone into the tank and shake the tank aggressively. Stop for a small bit and check if the rust is indeed coming out and resume the shaking process. Empty the acetone out into a sludge bucket and repeat the process.
Vinegar: Vinegar contains acetic acid, which helps in dissolving the rust from inside the tank. Pour vinegar into the empty tank till its brim. Close the tank and keep it overnight. The next morning, pour out some quantity of vinegar from the tank and shake the tank vigorously. Then empty the contents from the tank. Then pour a little baking soda into the tank and keep it aside for half an hour. Use water to rinse the tank.
Detergent: The other method to clean the fuel tank is to use laundry detergent combined with hot water. Leave the mixture in the tank for an hour or two. Then pour the mixture out and wash the tank with hot water again. If you are of the opinion that the tank is not clean as per your expectations, then you can repeat the process.
Acid: Muriatic acid or phosphoric acid can be used for the same. Like any other acid, care should be taken while handling muriatic or phosphoric acid. When using muriatic acid, remember to use thick rubber hand-gloves and glasses to protect your eyes from the acid fumes. Pour hot water in the tank till it is half full. Add approximately half a gallon of acid to the water. Then add in the remaining hot water in the tank. Close the gas tank with a cap, but make sure you do not use the tank cap. Do not disturb the tank for about 2 to 2 1/2 hours. Let the entire mixture settle in the tank. Dispose the mixture from the tank carefully.