Alkalinity: a measure of the ability of water to resist a decrease in pH upon the addition of an acid. The greater the number the greater the alkalinity (within a particular scale system). Alkalinity can be provided by any number of compounds (carbonate, bicarbonate, borate, phosphate, hydroxide). Thus a measure of alkalinity does not necessarily indicate the presence of any one of the above compounds.
KH:carbonate hardness. A measure of the amount of carbonate/bicarbonate in a given volume of water. In general this is typically the same as the alkalinity value, but it is not necessarily always so.

Temporary hardness: another name for KH. It is temporary because (as we read above) changes in CO₂ concentration and acid levels can rapidly affect this value. This is in contrast to GH (general hardness) which does not appreciably change in the short term.

General Hardness: a measure of the calcium and magnesium concentration.

SUMMARY

To fully explain how CO₂ and KH work in a planted aquarium you must understand the other topics here too. Atoms make up the bicarbonate molecule, which is held together by covalent and ionic bonds. When added to water the molecule will dissolve forming anions and cations and will set up an equilibrium between the carbonate, bicarbonate and carbonic acid forms. Changes in either pH, CO₂, bicarbonate or carbonate will influence the concentration of the other three. These changes occur because of LeChatelier's principle.

In a planted aquarium employing a properly set up CO₂ injection system the KH and pH should remain stable because one adds only as much CO₂ as the plants need, thereby maintaining a constant level of CO₂. In practice this is what you get when you use a CO₂ injection system with a pH feedback metering system.

Although you rarely have pH/KH problems when using this system, there does still exist a chemical route by which KH and pH can drop that cannot be remedied by a CO₂ system. This possibility exists because planted aquaria produce numerous acidifying organic compounds (e.g. tannic, humic, and uric acids) that will react with the KH (bicarbonate/carbonate) present. This reaction converts bicarbonate into carbonic acid. The carbonic acid produces CO₂ that can (i) be utilized, (ii) be gassed off, or (iii) reequilibrate (however the amount of bicarbonate produced is a tiny fraction of what was initially consumed). One would be alerted to this situation by a rapid drop in pH (which occurs when the bicarbonate concentration falls below the CO2 concentration). Once this occurs it is chemically impossible to restore this lost KH by maintaining or increasing the level of CO₂ injection.

This relationship becomes evident when we look at the equilibrium equation illustrated in Figure 4. For H₂CO₃ & HCO₃- the K value is 0.000000447. When we consider that the bicarbonate concentration is in the numerator and carbonic acid (CO₂) is in the denominator in this ratio it becomes clear that no matter how large the denominator becomes, the numerator will always be much, much smaller.

However, this drastic loss of KH is not something that commonly occurs because most people perform a water change well before that situation might arise. The water change water either already has a sufficient KH or can be brought to the appropriate level with the use of a bicarbonate based buffer. Although this is rare, it is important to be aware of lest anyone become complacent and forgo water changes for several months. How rapidly this process occurs is related to the overall cleanliness of the system (i.e. effective waste removal) and the initial KH.

In a well planted tank without CO₂ injection the plants will use the CO₂ available, causing the carbonates present to reequilibrate thereby producing more CO₂. The acidifying agents mentioned above will also tend to drive the bicarbonate equilibrium to produce more CO₂. In this type of setup the presence of acidifying agents can have a beneficial effect; it helps to maintain a higher level of CO₂ than would be present if utilization were the only driving force for the formation of additional CO₂. With this system one must actively maintain the KH by periodically adding bicarbonate based buffer. The plants are in essence 'eating' the buffer.

The plants’ use of CO₂ and the presence of acidifying agents in the water drive the bicarbonate equilibrium to maintain a constant level of CO₂. In most cases the amount of CO₂ produced is adequate for moderate to good levels of growth, however if the plants are capable of consuming CO₂ faster than reequilibration can produce it, then the reequilibration step will become the limiting factor in plant growth (assuming all other nutrients are at levels sufficient to not limit growth). The bicarbonate based buffering system maintains KH and pH because of LeChatelier's principle. Without an understanding of this basic chemical principle we would be unable to explain how one of the most basic systems in a planted aquarium functions.