There are many variables to consider when determining the weight of a 40 gallon tank. The weight will depend on the material the tank is made from, whether it is empty or full, and what type of liquid is inside.
A 40 gallon tank made from plastic will weigh significantly less than one made from steel. An empty tank will also weigh less than a full tank. And, finally, water weighs more than most other liquids, so a 40 gallon water tank will be heavier than a 40 gallon gasoline tank.
To give you a rough idea, an empty plastic 40 gallon tanks weighs about 30 pounds, while a full steel 40 gallon tanks can weigh over 400 pounds.