it is not a straight forward thing so a straight forward answer is nigh on impossible but I have done a lot of research into this and have run CO2 in the past so here goes.
Using a presurised CO2 cylinder there are three ways of doing this. First is to just have the CO2 bubbling into the tank at a constant rate. This will help the plant grow a bit but you with either be running at low concentrations of CO2 which makes the whole exercise useless or causing huge fluctuations in the pH level and killing your plants and fish.
The second option is to go with a solinoid setup which will turn the flow of gas off and on. this is generally attached to a timer that comes on an hour after lights on and shuts off half an hour before lights off. This means that the CO2 is only pumping when the plants need it. this will reduce the pH swing but not eliminate it. Again you will be aiming to run the CO2 at a lower concentration than optimum as you do not want to risk running it too high.
The final option is to attach the solinoid to a pH controler which will regulate when the CO2 is pumping. As the pH drops too low the controller will turn the gas off, as the plants use the CO2 and the pH rises it will switch it back on again. This allows you to maintain the optimum pH concentration without risk to your fish.
I never recommend anything but number 3 as even with that there are still pitfals and you can easily kill off an entire tank of fish. And i am not just running my mouth here, i lost $100s of discus when i first started as i measured my KH wrong and set the pH too low.
The other thing is that fire extinguishers do not use food grade CO2. They use a very crude CO2 that is full of impurities and who knows what else. I would be using food grade CO2 avaliable from most big aquarium stores or better (and cheaper) from home brewing places. Much better stuff. Not to mention beer tastings
These days it is quite cheap to set up a fully automatic CO2 system. i set mine up for $750ish with two bottles and an external reactor and like i said I would be going this way or not at all. DIY CO2 can be fun and does work but it is only ever a low concentration of CO2 which is good for a plant boost but not for the full glory of CO2.
Hope that helps
Doug's Bristlenose eBook -
The number one source of Bristlenose information and the most detailed breeding guide avaliable
3rd edition LOADS OF INFORMATION, PICS, DIAGRAMS AND BRISTLENOSE GOODNESS
GO TO www.keepingandbreedingbristlenose.com FOR YOUR COPY!!
Like my book - why not "Like" it on Facebook!
Click here to vote for Bristlenose World