How big are the tanks in soda fountains?? You could say you're a serious home-brewer.. Its a cleaner way to carbonate beer for those who don't like home-brew sediment..
If you are going to go with a story like that though, study up on the excuse.. The last thing you want is for the guy to be interested in brewing himself, and wanna chat if you know nothing about beer making.. Actually high-end brewing can get VERY complex.. I suppose you could say you're picking up the tank for somebody else who brews..
Don't jump into excuses though, just have them ready incase the guy actually asks.. There is nothing more obvious that somebody too eager to shoot off a BS story.. With any luck he'll know the game, and just hand you the bottle..
Thats what I always found when I bought HIDs from contractor outfitters.. You'd think a university kid buying lots of high-bay lighting would warrant some curiousity, but they were always smart/polite enough to know that I didn't want to be put on the spot regarding a grow op..
The only time I ever even had a conversation with the guy was the first time I bought them.. I needed cords that didn't come with the bare-bones units, and asked what they had in stock.. The guys reply was "Well, if you aren't going to need to pass code inspection the cheapest way it to just cut up some extension cords.."
He knew what I was up to.. He was rooting for me though I could tell.. And there was no way to identify me since I didn't drive there, and payed cash, and I had a nifty way to get them home safely just incase..