In certain situations, you need to add extra carbon dioxide to aquarium water. Carbon dioxide diffusers allow you to bubble this extra carbon dioxide in. Several different versions of diffusers are available, but all work on the same principles.
Carbon Dioxide and Freshwater Aquariums
Planted freshwater aquariums often need extra carbon dioxide. Under ideal conditions, plants absorb carbon dioxide and release oxygen. However, the ambient carbon dioxide in most aquariums cannot support robust plant growth. Generally aquarium plants need about 15 to 30 ppm (parts per million) of carbon dioxide to thrive. Though carbon dioxide can be toxic to animals in high concentrations, it is harmless at levels below 40 ppm.
Unfortunately, getting carbon dioxide dissolved into an aquarium takes a bit of ingenuity. Since carbon dioxide is a gas, you can't just bubble it through water to mix it in—at least not in sufficient levels to support vigorous plant growth. Diffusers use various physics tricks to increase the diffusion of carbon dioxide into aquarium water. They do this in a number of ways, including actually increasing the surface area of water exposed to carbon dioxide, agitating water to make it absorb carbon dioxide more readily and keeping carbon dioxide close to water for extended periods. Different diffuser designs employ one or more of these techniques.
Types of Diffusers
The simplest diffuser the air stone. Air stones are made of a porous material that breaks up a steady stream of carbon dioxide into tiny bubbles. Very small bubbles have a much higher surface-area-to-volume ratio than large ones, making it easier for the carbon dioxide to be diffused into the water. However, this method is not very efficient. Baffle systems use various shapes—usually spirals—to increase the amount of time it takes carbon dioxide to travel through the water column. Bell-type diffusers hold carbon dioxide gas in bell-shaped vessels under the waterline of aquariums to give it a better chance of diffusing. Lastly, carbon dioxide reactors use agitation to mix aquarium water and carbon dioxide, usually within a cylinder-shaped container.
In marine aquariums, calcium reactors use carbon dioxide diffusers to lower the water's pH within the reactor. This makes it easier to dissolve calcium and bicarbonate into the water, to be absorbed by corals and other invertebrates. When water leaves a calcium reactor, the carbon dioxide evaporates, leaving the water rich with dissolved minerals.