If the pressure drops it is because there is no liquid left and therefore it is time to replace the cylinder.
If the tank pressure drops, yes the tank is near the end of its life. However, past a certain pressure point while gas is still in the tank and using a single stage regulator, the remaining CO2 in the tank will dump past the metering valve (because of the low pressure) into the calcium reactor turning your calcium media to mush sending super high alkalinity effluent into your system with the potential to nuke your tank. Hence the term, end of tank dump. And in case you’re wondering... by the time the pH probe reads the drop in pH in your reactor and closes the solenoid, it’s too late.
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