Yeast

From The Aquarium Wiki

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.

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

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 yeast.