A type of microscopic fungus, which reproduces by budding or cell division and will convert sugar into alcohol and carbon dioxide. Bakers yeast, a yeast preparation used for raising bread and cooking, can be used to create a carbon dioxide dosing system for planted aquariums. This is normally done by filling a container with roughly a quarter sugar, two quarters water, a heaped teaspoon of bakers yeast, and a teaspoon of bicarbonate of soda, leaving about a quarter of the container filled with air. Over a 24-hour period, a reaction starts to take place, which steadily produces carbon dioxide. By attaching an airline from the sealed container to the aquarium, the carbon dioxide is forced through the airline and into the tank. For more effective dissolving of the carbon dioxide into the water, the airline can be attached to a pump (venturi) or a bubble counter, allowing a greater contact time between the aquarium water and carbon dioxide. This type of system normally lasts for a few weeks before needing re-filling.