I am attempting a CO2 system on my 58 gallon tank.. I am planning on having it turn off at night. The local Gas store only has 5 and 20 lbs tanks... originally I was planning on setting up a 10lb system. Now I am not sure if I should move down or up. How long does a tank usually last (average)... is it worth going up to the 20lbs or just putting a 5lbs tank?