Elements of the Aquarium - Carbon

C - Carbon
CaCO3- - Bicarbonate (Calcium Bicarbonate)
CO2 - Carbon Dioxide
CO32- - Carbonate
H2CO3 - Carbonic Acid

Carbon is a very abundant element found in the aquarium system and not just as activated charcoal media for filters.  Listed below are some of the more note-worthy compounds that carbon atoms take place in.

Carbon is present in the aquarium as a byproduct of the respiration of fish, invertebrates, and aquatic plants given off as carbon dioxide (CO2).  Photosynthesizing plants also require carbon which they get in the form of carbonic acid (H2CO3).  Carbonic acid is a chemical combination of CO2 and water (H2O).  A lack of available carbon usually causes the leaves of a plant to yellow and the growth process of that plant will slow.

Though the presence of carbon dioxide in the aquarium is inevitable and in a planted aquarium even a good thing, if there is too much carbon dioxide in the tank, the fish and invertebrates will suffer.

The pH level of the aquarium water is a concern for most aquarists and carbon, in the form of carbon dioxide, has an effect on this as well.  pH is a measure of how acidic or basic the water is.  Specifically, it is the concentration of hydrogen ions (H+) in the aquarium water.  A high count of hydrogen ions defines the pH level as being low or acidic and a low H+ count means that the pH level is high or basic.

During the hours where enough light is available in the aquarium, photosynthesis in plants and algae uses CO2 and releases oxygen.  This exchange increases the pH level because of the lower concentration of dissolved carbon dioxide in the water.  Then, during the hours without enough light, plants respire.  Respiration uses dissolved oxygen and releases CO2.  Thus, the dissolved CO2 concentration rises and some of that carbon dioxide dissolved in the water forms carbonic acid (H2CO3), which raises the pH and makes the water more basic.

pH should not be confused with alkalinity.  These two items are related, but are not the same. Alkalinity actually refers to the capability of water to maintain stable pH levels.  This is called the buffering capacity of the water.  If there is a problem with the buffing capacity of the water, a chemical called a buffer can be added to the tank.  A buffer is a chemical that allows a solution to resist changes in pH.  Buffers are often weak acids that partially ionize in an acetic acid (CH3COOH) solution, a solution that also contains carbon.  Alternatively, there are buffers that will also lower the pH of the water.  The main buffers found in the aquarium hobby are carbonate (CO3) and bicarbonate (CaCO3) also called calcium bicarbonate.

As mentioned above carbon plays a large roll as calcium carbonate (CaCO3), not only as a buffer additive, but also as a way to measure the carbonate hardness (kH) of the aquarium water.  This type of hardness is depicted by how many parts per million (ppm) of carbonate ions (introduced as either CO3 or CaCO3) there are dissolved in the water.  This then, usually translates into measurements of kH.  Though, kH is not used nearly as often as dH (degrees of hardness) and gH (general hardness) which measure all of the minerals found in the water, most notably calcium (Ca, which may have been introduced as CaCO3) and magnesium (Mg).  So, water with a high hardness measurement (high levels of carbonate ions) will also have a very high buffering capacity and a very basic pH level.  Especially soft water can actually cause problems because softer water has less buffering capacity due to a lack of minerals.  This usually leads to drops or swings in pH which can easily cause harm to the aquarium inhabitants.

All carbon that is found in the aquarium system stays in the system unless it is removed by regular water changes.

If needed carbon dioxide can be added to the systems by lowering the water hardness to free CO2 from calcium bicarbonate.  CO2 can also be added by the use of a CO2 system or other organic carbon compound.

Electron Shell Diagram

Carbon electron shell diagram.



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