Consider 2x 2 liter bottles and switching one out every other week, it will keep your co2 consistent. I use 2x 2 liter bottles for my 15 gal and they give me 1-1.5 bps, my drop checker stays green/yellow green. My home stays around 73 to 74 f and 2 cups sugar, 1 tsp yeast and 1 tsp baking soda last 2 weeks. Do make sure you tube the 2 liters to a watter bottle half filled or similar before you run the tube to a power head or diffuser, if not youll get nasty yeast growth in your tank. The water bottle acts as a gas separation chamber and "scrubs" the co2 bubbles so that only the gas reaches your aquarium and not air born yeast. Im sure you already know to use a check valve to prevent the tank from siphoning back into the 2 liters. I use the small fine bubble air stones from the pet store for a diffuser. I ziptied mine to the underside of a koralia type power head and my tank is FILLED with tiny co2 bubbles. Make sure you keep the waters surface calm, surface agitation gasses off co2. Fluval drop checkers are cheap and helpful to let you know where your co2 is.