We rigged up a pretty good little system with a CO2 tank from a beer tap, a solenoid valve from active surplus, a lamp timer and some perforated surgical tubing. We set it so it would go off for 15 minute periods twice a day, controlling the total flow with the main tank valve. We made sure not to enter the room during or immediately after releasing the CO2 as we didn't want to suffocate (even though we weren't pumping that much out.. safety first!) When we ran out of C02 we just didn't bother getting a refill and just went with 2 litre bottles filled with sugar, yeast and water. Its a safer, cheaper setup, but can be messy and requires feeding and shaking every now and then. Also natural, but generates new CO2 compared to CO2 tanks which generally use recycled CO2...