Perhaps you should use white granulated sugar. I used to get 11-13 days on a bottle of DIY CO2. 3/4 cup sugar, 1 teaspoon yeast, rest lukewarm water. I used dry yeast, a rapid-rise brand. However, always made sure it was fresh (take notice of date when purchasing it, reach for packets toward the back of supermarket shelves). On my 120 gal I had two DIY CO2 generators. That allowed me to use 2 teaspoons of a fresh package of dry yeast each time. (yes, I would throw some away.) My experience was if I used what remained in the packet at some later time I would only get about 7-8 days of CO2 production.
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