[S. cerevisiae]

I have owned steel (20lb) and aluminum (5 and 10 pound) CO2 tanks.  My back prefers aluminum, but steel tanks win hands down when it comes to longevity.  I am still amazed by the fact that my original steel CO2 tank had an original test date in the late 40s.  I have never seen an aluminum tank with a really old original hydrostatic test date.

With that said, my favorite tank size from a use point of view is 5 pounds.  However, 20lb tanks are the clear winner when it comes to total cost of ownership.

[denny]

I've got a steel 20 lb.  Yeah, it's heavy.  I now have kegs in 2 different locations, so I'm thinking of picking up a 5 lb. also.

[Thirsty_Monk]

It is a gas what we are after. Tank does not matter too much as long as it holds liquid gas.

[brewinhard]

I prefer my 20#'er as I typically only have to fill it once a year and it lasts at least that long most of the time.  I only keg my beer, use the tank for my beer gun for comps and to close transfer when packaging into the keg.

[jeffy]

I keep a 20 pounder on hand and at least one 5 pound bottle just in case I'm stupid and run out.  The small bottle is handy for other uses in the brewery.

[tommymorris]

I exchange my tanks at airgas. They try to give me Aluminum if they have one. They usually don't. I don't care and I never ask, but, the folks there always tell me they tried to find Aluminum. 

5# last me at least 6 months maybe longer.

[hopfenundmalz]

I have kegs in the garage and in the basement. I have a 5, a 10, and 2 of the 20s. One 20 is steel, the other AL.

The 5 and 10 seem to spend more time in the basement due to the stairs. The AL 20 May go down there is needed, but not the steel one.

[ajk]

Lately, local establishments have stopped exchanging 20# tanks, so now I use a 15. I'd like to get a little paintball tank for traveling and emergencies.

[S. cerevisiae]

I go through around 5 pounds of CO2 per year.   However, I am not into gassy beer, and most of my soda kegs are 3-gallon kegs (my standard batch size is 3.5 gallons). 

[klickitat jim]

I have two 5s. They last several months unless I spring a leak... I would like to add a 20 but I'm trying to imagine how irritated I would be to find a full 20lbs empty overnight because of a stupid o ring.

[mainebrewer]

I have two 5# AL tanks, one as a spare. They last an average of 6 mos. This is carbing/serving 5 kegs, pushing cleaner/sanitizer through kegs and beer lines, etc. While the refill cost/pound is cheaper on the 20#, I just like the size and weight of the 5#.

[Slowbrew]

I have two 20 pounders and a 2.5 pound for the party cooler.

Paul

[theDarkSide]

I have a 20 lb. tank for the kegerator and a 5 lb. tank for when the 20 runs out on a Sunday afternoon after everyone is closed. 

[narvin]

Just got a 20lb tank for the downstairs lagering fridge.  It was free from a homebrewing friend who was doing work on some old union shop and found 10 of them in the basement that the owners wanted to get rid of.

I have a 5lb tank for my kegerator built into a wooden bar, since it fits in the fridge.  Also use another 5lb tank as a backup, for transfering out of the fermenter under pressure, and for my serial killer bolt gun murder apparatus.

[dkfick]

I have a 50lb tank, a 20lb tank, and several 5lb tanks.  I use the 20lb tank is probably the best at being portable yet cheapest for the amount of co2 you get.  Of course the 50lb tank is cheap to get filled but I am not moving it around anywhere... The tare weight on it is 150lbs (previous tank was 100lbs... didn't notice this one was so much heavier last time I exchanged it)