Nothing except possibly a fuel stabilizer if you're going to store the vehicle. You may also want to add a good fuel injector cleaner such as Seafoam or Techron once a year.
Anything other than gasoline
Water is heavier than gasoline, it will pool at the lowest point of the fuel tank.
Unless you are shortening gasoline, then no. Gasoline: "I put a lot of gas in my tank." Other: "Oxygen and helium are gases."
That could be a gas tank or a gas can...
Inside the gas tank
With gasoline the fumes are flammable, not the actual liquid, so the newly emptied gas tank would combust rapidly, and the completely full tank would catch fire and burn longer, but it wouldn't be as bad as the newly emptied gas tank
The Gas Tank holds 61 liters, or 16.1 gallons of gasoline.
The fuel level gauge will most likely read past the full mark in the dash panel. Depending on your level of overfilling, gasoline will spill out of the reservoir spout as you fill the tank. Other than the gasoline on the paint of your car and the ground, no harm done to your vehicle.
The tank holds 18 gallons of gasoline. Check the manual for other capacities such as coolant, oil etc.
It is hard to know when you have water in your gas tank. However, that is no longer the problem that it used to be in the United States. Most of the gasoline in the United States contains alcohol. The alcohol makes the water mix with the gasoline. As water condenses in the gas tank it mixes with the gasoline and is removed in the engine exhaust.
Yes, you can siphon gas out of a 1991 Buick LeSabre, which is handy if you happen to put diesel into the tank rather than gasoline. Just use a hose with a pump or some type of suction device to remove the bad gasoline from the tank.
In the gas tank...put one bottle to a full tank of gasoline.