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Yeasts are microorganisms classified as Fungi. Over seven hundred species of yeasts have been described.

In the aquarium hobby the species of yeast often used is Saccharomyces cerevisiae, this yeast has been used in brewing and baking for thousands of years, and is also extremely important as a model organism in modern cell biology research. In the aquarium hobby we use this yeast to make CO2, to feed infusoria or to feed certain worm cultures like micro-worms. This item is widely available in the local shop and is usually very cheap.

It comes in a powder like form wrapped in silver paper lined packets to keep the yeast in a dormant state and it can last for a year in this way.

Making CO2[edit]

The yeast is simply added to sugar with warm dechlorinated water (around 20°C (68°F) ) to start the CO2 reaction. A by product of this reaction is that alcohol is produced and this eventually causes a pH shift which kills the yeast. So add Sodium bicarbonate to delay this shift and therefore produce CO2 for longer.

  • Leave only a small air gap between the water surface of the yeast bottle and the bottle cap. Yeast fermenting is a anaerobic reaction and so before it can start producing CO2 it needs to use up all the oxygen in the top of the yeast bottle and the hose. Ensure there are no air leaks in the hose or at its junction to the bottle or the CO2 reaction may not start.
  • ¼ tsp of baker yeast followed by 4 tbsp of sugar and 1 tbsp of Sodium bicarbonate is all that is usually required.
  • Companies have developed different strains of yeast which are more tolerant of the presence of the alcohol and one such strain is said to be Champagne or Wine yeast. If you use these, add more sugar. For a list of high alcohol tolerant yeasts see this page of Yeasts available in the Wine making trade.
  • Use a Yeast nutrient like Fermaid K or Tronozymol (if you're in the UK) to ensure the yeast gets a good start in life and has the necessary building blocks to give you a good steady supply of CO2. Alternatively if you try a small amount (½ tsp) of Malt Extract such as Marmite spread or a squirt of Lemon juice to the mixture then this can act as a yeast nutrient.
  • Keep the unused yeast in a sealed dry container in the Fridge to make it last longer. It doesn't like exposure to air or the damp. It doesn't last forever though, to test old yeast, place a small amount in ~20-25°C (68-77°F) water and after 15 minutes there should be foam on the surface.