Another way to figure out how many gallons is in a tank is to measure the tank (if you can get to it, that is) using foot measurements. Multiply all three sides (H X W X L). That will give you the cubic feet of the tank. Then multiply by 7.48 and that will give you a close approximation of the number of gallons that a particular tank will hold.
ei. you have a tank that measures 4'L by 1.5'H by 1.5'W. 4 X 1.5 X 1.5 = 9 cubic feet. 9 X 7.48 (gallons per cubic foot) = 67.32. So that tank holds about 67 or so gallons.
I know, not all tanks are perfectly straight on the edges, but then, one isn't looking for an exact figure. The measurements will give you a close approximation of the number of gallons a tank will hold.