Do not fill it with HPA! Co2 pressures usually range in the area of 850-1200psi. Bad things can happen if you over pressurize a tank.You really don't want to be going there. Google your question on how to fill. I usually cool my tank in the freezer for a fifteen minutes before filling it to get it to accept co2 more easily. I have never been able to overfill a tank. Always weigh your tank before, to see that it is empty and after to be sure it is not over filled.