What size co2 tank should get?

I had a pressurized co2 system, with a 5lb tank running for about 5 months. I ran 2 bubbles per second in my 36 gallon tank, and never emptied it. Here are some co2 resources from Rex's site:

http://www.rexgrigg.com/CO Resource.html
 
I have a 100gal with CO2. I have the LARGE 20lb bottle and it last me about a year (10-12 months).
 
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