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I was on the ontariobeerkegs website and saw this
http://www.ontariobeerkegs.com/product_p/carb5.htm" onclick="window.open(this.href);return false;

do these really carbonate your beer overnight? has anyone ever tried them?

I have one, and a lid to use it. I don't bother anymore, as I have enough beer stockpiled that I`m not in a rush, but IIRC, when I used it, it worked good enough. I use the stone for O2 additions now.