You could change it into a holding tank.
Not likely a fuel tank. Very few water tanks can be used as fuel tanks, although many fuel tanks seem to do well at being water tanks...sorry just a joke about dirty fuel tanks.
You might be able to put a fuel bladder inside the tank to make it a fuel tank.
Have faith that the oceans are going to rise and flood the world, that plague and pestilence brought on by Climate Change is going to punish us for not believing. Please do as they say it is our only hope. :P