are you using bakers yeast or champagne yeast. currently i've got bakers yeast going as a trial but i seem to remember a post saying that champagne yeast can survice the alocoholic conditions in the DIY co2 environment. must be tough stuff - i sometimes feel like i don't survive tequila conditions too well!
i'm only running a 1.25 lt bottle for my little tank - about 20 bubbles per minute at the moment.
Toronto - man you must need some heaters there