I used the teta and it was useless.
I have a 20gal and I use a diy yeast kit and my drop checker is always green - thanks to all the help from the good folk on this site!
2 x 2 litre coke bottles hooked up to a single CO2 line which I have placed in my filter uptake pipe. I change 1 bottle over every 5 days to keep the CO2 stable. It costs a few pence to make and run.
Let me know if you need more info.
Steve