The Specific Gravity of a substance is a measurement of the weight per gallon of that item compared to the weight of water. Water weighs 8.345404 pounds per gallon and it has a specific gravity of 1. A substance that weighs 1-1/2 times as much as water will have a specific gravity of 1.5.
Most plastic tanks are rated by the specific gravity of the substances they are designed to hold. Ratings of 1.5 specific gravity for general purpose tanks and 1.9 specific gravity for industrial grade tanks are common. Some producers try and bridge the gap with 1.7 specific gravity tanks.
You should always know the specific gravity of a substance in order to purchase the correct grade of tank. Storing too heavy a substance in a lighter weight tank can cause the tank to fail.