Question: What do aquarium hobbyists use yeast to generate?
About the article "Yeast", section "Aquarium hobby".

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Yeast is often used by aquarium hobbyists to generate carbon dioxide (CO2) to nourish plants in planted aquaria.
A homemade setup is widely used as a cheap and simple alternative to pressurized CO2 systems.
While not as effective as these, the homemade setup is considerably cheaper for less-demanding hobbyists.
Several recipes for homemade CO2 are available, but they are variations of the basic recipe: Baker's yeast, with sugar, baking soda, and water, are added to a plastic bottle.
A few drops of vegetable oil at the start reduces surface tension and speeds the release of CO2.
This will produce CO2 for about two or three weeks; the use of a bubble counter determines production.
The CO2 is injected in the aquarium through a narrow hose and released through a diffuser that helps dissolve the gas in the water.
The CO2 is used by plants in the photosynthesis process.
In addition to design, freshwater aquascaping also requires specific methods to maintain healthy plants underwater.
Plants are often trimmed to obtain the desired shape, and they can be positioned by tying them in place inconspicuously with thread.
Most serious aquascapers use aquarium-safe fertilizers, commonly in liquid or tablet form, to help the plants fill out more rapidly.
Some aquarium substrates containing laterite also provide nutrients.
It is also necessary to support photosynthesis, by providing light and carbon dioxide.
A variety of lighting systems may be used to produce the full spectrum of light, usually at 2–4 watts per gallon (0.5–1 watts per litre).
Lights are usually controlled by a timer that allows the plants to be acclimated to a set cycle.
Depending on the number of plants and fish, the aquascape may also require carbon dioxide supplementation.
This can be accomplished with a simple homemade system, using a soda bottle filled with yeast, warm water, and sugar, and connected to an airstone in the aquarium, or with a pressurized CO2 tank that injects a set amount of carbon dioxide into the aquarium water.
Algae (including cyanobacteria, as well as true algae) is considered distracting and unwanted in aquascaping, and is controlled in several ways.
One is the use of animals that consume algae, such as some fish (notably cyprinids of the genera "Crossocheilus" and "Gyrinocheilus", and catfish of the genera "Ancistrus", "Hypostomus", and "Otocinclus"), shrimp, or snails, to clean the algae that collects on the leaves.
A second is using adequate light and CO2 to promote rapid growth of desired plants, while controlling nutrient levels, to ensure that the plants utilize all fertilizer without leaving nutrients to support algae.
Although serious aquascapers often use a considerable amount of equipment to provide lighting, filtration, and CO2 supplementation to the tank, some hobbyists choose instead to maintain plants with a minimum of technology, and some have reported success in producing lush plant growth this way.
This approach, sometimes called the "natural planted tank" and popularized by Diana Walstad, can include the use of soil in place of aquarium gravel, the elimination of CO2 apparatus and most filtration, and limited lighting.
Instead, only a few fish are kept, to limit the quantity of fish waste, and the plants themselves are used to perform the water-cleansing role typically played by aquarium filters, by utilizing what fish waste there is as fertilizer. 
"Caulerpa" is common in the aquarium hobby as a nitrate absorber because of its rapid growth under relatively adverse conditions.
It may also be used in refugiums for a long term nitrite absorber.
Many introductions of invasive "Caulerpa" to the wild are thought to have occurred via aquarium dumping although there is no proof that this is so.
For this reason, some aquarium hobbyists have begun using Chaetomorpha or an Algae scrubber instead. 
Many hobbyists believe that a 30-gallon tank is best for one fish or 60 gallons for a pair.
In the wild, it is strictly associated with the sea anemone "Entacmaea quadricolor", and thus many hobbyists provide this species in addition to the fish.
The maroon clownfish likes frozen shrimp and herbivore preparations.
Many hobbyists do not catch the fish using a net.
The spines may get entangled in the net, which can injure the fish.
Instead, hobbyists tend to use a cup.
The maroon clownfish is one of the larger, more aggressive members of the clown family.
Consequently, they are typically housed singly, the only exception being a mated pair.
To avoid aggression, maroon clownfish are not normally mixed with any other type of clownfish, and the rock work is rearranged periodically.
Also, they are often the last fish added to a tank.
They are human-responsive to the point of trying to "intimidate" people with whom they are not familiar.
The fish has successfully bred in a home aquarium. 
The recommended tank size for housing is a minimum of 10 gallons.
A 10 gallon aquarium can keep 1 male with up to 2 females; however there is still a chance of confrontation that could end in a fatal fight.
A sandy substrate is ideal to allow for easy foraging and to facilitate burrowing habits.
Several hiding places should be provided; the use of driftwood, aquarium décor, and aquatic plants (real or plastic/silk) are ideal.
Despite what many hobbyists may say, Red Claw Crabs are not fully aquatic and must have access to land.
This can be done by having a half-land-half-water style aquarium set-up, or by having aquarium decorations that are taller than the water line.
These crabs are excellent climbers and great escape artists.
Therefore, the water line should be at least a few inches below the rim of the tank, and a tightly fitting, fully sealed lid should always be in place.
The water should be cycled and properly filtered.
A weekly 10% water change can further improve water quality.