Planted Tank Enthusiast
Join Date: Apr 2004
Location: Des Moines, Iowa
Two things to add to scolley's information.
1) You have to remember to keep up your tanks buffering capacity (KH), otherwise, the pH will remain constant, the KH will ultimately decrease and you won't be getting the CO2 levels you want. generally, good regular water changes are sufficient unless your tap water doesn't have much buffering capacity to begin with.
2) End of tank dump occurs when the remaining pressure in the tank isn't sufficient enough for proper regulator operation (there needs to be a certain amount of pressure on the regulators diaphragm I believe) and the regulator essentially opens up and any remaining gas in the tank is dumped out into the system. In a system without a controller, there is the potential for almost immediate disaster from the massive pH swing the tank will experience and possible CO2 poisoning. In a system with a controlled regulator, the danger isn't as crtical because as soon as the pH goes below the set point, the solenoid valve closes and the Milwaukke solenoid valve (as I am sure are other brands) can easily handle the relatively low pressures that are present at EOTD.
That being said of course, monitoring the tank pressure will tell you when the liquid CO2 is gone and only gas remains, at which point the pressure in the tank will gradually start decrease from it's nominal pressure (usually around 850-1000 for CO2)
at which point you need to plan on getting a refill. I usually refill mine at about 400-500PSI although I have let it get as low as 300.