Commercial tank manufacturers rate their tanks by outside measurements. So figure z(length (inches) x width (inches) x height (inches))/ 277.4 gallon/inches^3. So (72 x 36 x 36)/277.4 = 336.38 gallons
Being a custom acrylic tank it was probably designed to be a 300 gallon tank using inside dimensions for sizing and for a water level not all the way to the top edge, therefore it is probably a 300 gallon tank. BIG TANK.