A fish tank is normally a rectangular prism. To find the volume of the tank, use the volume formula of a rectangular prism. The formula is length times width times height. The volume of the tank is how much water the tank will hold. This is important for knowing how much of a chemical to put into the water and to determine the size of the tank.
Measure from the top of the tank to the bottom of the tank. This is the height. For example, a tank has a height of 2 feet.
Measure from one end of the tank to the opposite end of the tank. This is the length. In the example, the tank has a length of 4 feet.
Measure from the front of the tank to the back. This is the width. In the example, the width of the tank is 1.5 feet.
Multiply the tank's width by the height and length. In the example, 2 feet * 4 feet * 1.5 feet equals 12 cubic feet.