are of a triangle is 1/2 X base X the height (of the triangle, not the aquarium). To get the volume multiply that times the height of the aquarium (all this in inches). To get the gallons divide by 231.
Height of the triangle is measured from the base to the angle opposite the base.
So...1/2 X B X HofT X HofA / 231= gallons Just make sure all your measurments are in inches.