Pierce CO2 system

Indeed, most of these small cartridge typed CO2 systems have "hidden" costs (such as the disposable cartrdiges that you mentioned).

If you want to get a refillable one, you will need an adapter from the regulator to a more common fitting (i.e. the paintball pin-type fitting, or even a CGA320 fitting for a normal CO2 cylinder).

I bought a pierce system and its ok for smaller tanks. For a 20 gallon I would suggest buying a 5lb CO2 tank or paintball canister along with a used regulator on ebay. Within half a year you would already be saving money over the pierce setup.

Co2 tank lasted less than 2 weeks at 30 bubbles per minute. Very difficult to set that flow rate that slow. Definatley not cost effective