It just threw off my water stats... sometimes I would get a ph jump. I had one 5 lb tank, and the only place I could get it filled was at a fire extinguisher inspection store. It was $10 a fill, and lasted about a month and a half I believe... it's been a while. They were a 9-5 store, and I work shifts, so if I was 7-3 or 3-11, I'd take it on my way to/from work, so it would only be 10/12 hours with out co2, but if I was on a week of 9-5, I would be up to 5 days, unless I could bribe my parents to head down there. A few times I did do a sugar/yeast bottle to at least have something in there, even though it was nowhere near what it usually got.
Somewhere to check out would be at a fire station - they *should* be able to recommend places to find and fill your tank.
ps... All this is really making me want to get back into co2... eep! Must resist!