Not log.
It really depends on how many bubble per second you had your setup
running. It is MUCH cheaper in the long run to get the largest tank
that will fit in your space.
A 5 lb tank refill may cost 22 while a 2.5 lb may cost 20 for a
refill.

Marc

A 10lb tank cost about $100 and CO2 refills run about $12 for the 10lb
tank here in Spokane, ( our source from a good welding supply
company.)

Steverd