The CO2 in your tank is in liquid form. As long as there's ANY liquid left, the pressure will be the same, usually around 850psi plus or minus. The value is dependent on temperature. The liquid CO2 evaporates into the CO2 gas that you're sending into your aquarium. Once ALL of the liquid is gone, the pressure will start to decrease fairly rapidly as there is no liquid CO2 left in the tank to maintain that pressure. When the pressure reading starts to decrease, you're actually petty close to empty!