The needle valve, however good it is, will not stop an "end of tank dump", which happens with cheap regulators that let the output pressure go up as the input pressure goes down. A pH controller should shut off the CO2 long before that end of tank dump can raise the CO2 level enough to harm the fish. You can avoid the end of tank dump by monitoring the CO2 tank pressure - the reading on the high pressure gage on the regulator. When that pressure drops to 500-600psi, refill the CO2 tank, because there is no liquid CO2 left in it. To do this you have to look at the pressure about twice a day.