CARBONATE HARDNESS

The molecule responsible for stabilizing the pH against these influences is called the carbonate molecule. A measurement of carbonate molecules is expressed as the total alkalinity (TA), or the carbonate hardness (KH).
Carbonates come from several places. In nature they come from the slow dissolution of natural minerals in rocks. In particular, limestone and gypsum are rich in carbonates, as are seashells and corals. As they dissolve, they release minerals such as calcium and magnesium, as well as carbonate molecules, into the water.
The carbonate molecule exists in a balance with the environment. When hydrogen ions become abundant, such as through biological processes, the carbonate molecules pick up the extra, which prevents the pH from falling. When hydrogen ions become scarce, the carbonate molecules will liberate some hydrogen ions. The net effect of the carbonate molecules on the water is to hold the pH at some constant level, which is why they are often called a buffer or buffering agent. Because of the importance of this buffering capacity of carbonate molecules, there is a benefit in having a quantitative measurement of carbonate activity in your aquarium water.
The carbonate levels in a system are measured by a test of the total alkalinity. Most major garden centers, pet shops, and pool-supply stores carry affordable total alkalinity test strips. Test results will vary depending upon regional conditions. For example, water in the eastern United States tends to have a total alkalinity below 50 PPM, while in the southeastern United States, the average total alkalinity is less than 30 PPM. In the southwestern United States, the water in the aquifer is mostly bedded in limestone or is derived from evaporation, and so the water has a very high total alkalinity of over 180 PPM.
A high total alkalinity E100 PPM and above Ewill keep your pH stable for along period of time. A low total alkalinity E50 PPM or under Ewill need to be remedied or you may have to cope with a sagging pH or sudden drop in ph. This will be discussed in more detail shortly. It is also possible to have a total alkalinity that is too high. A total alkalinity higher than 300 PPM may cause gill damage.
Not only do carbonate molecules occur in varying amounts in the environment, but they are also an exhaustible resource. When the carbonates are exhausted, the effect is a sudden drop in pH, which can and does kill fish.
A pH “crashEis a quick way to rid your tanks of all those messy fish. Here’s how to scenario often unfolds in the hobbyist’s tank. The water was changed two weeks ago, and at that time a satisfactory amount of carbonates existed in the system. The fish are fed daily, and the filter reduces their nitrogenous wastes. The hydrogen ions are bound up by the carbonates and all is well. The hobbyist is lulled into a false sense of security because the pH has been stable for weeks. Why check it now? Then the carbonates are finally exhausted, and overnight the fishesEcarbon dioxide production, the algae’s carbon dioxide production, and the reduction of the ammonia in the filter crashes the pH to 5.5 and the collection is all but lost.
I advise all hobbyists with water of low total alkalinity to use a good commercial buffer that will keep the pH at near neutral and the total alkalinity at about 100 PPM on a weekly basis.