A fuel tank can tip over or float in a flood, causing fuel to spill or catch fire. Cleaning up a house that has been inundated with flood waters containing fuel oil can be extremely difficult and costly.
Fuel tanks should be securely anchored to the floor. Make sure vents and fill line openings are above projected flood levels.
Propane tanks are the property of the propane company. You will need written permission to anchor them. Ask whether the company can do it first.
Make sure all work conforms to state and local building codes.