Morning Jun,
I copied the paragraphs from the krib (
http://www.thekrib.com/Plants/CO2/) as my science knowledge sketchy at best
Carbonate and General Hardness
Many beginners (and even advanced aquarists) get the different forms of ``water hardness'' confused. Here are two nice articles to help:
detailed article that summarizes many of the details presented here, in a single concise manner, and
a somewhat simpler one with a historical perspective on the units. See also
A little article from the old FAQ. KH, or
carbonate hardness refers to the concentration of carbonates, ie CaCO3. GH, or
general hardness, refers to dissolved calcium and magnesium. In many cases, these values are similar, because the minerals in your water are dissolved calcium carbonate. In the context of fish liking hard vs. soft water, we speak of GH. However, in discussing CO2, we will be dealing with KH.
Before delving into CO2 proper (and alienating the rest of the aquaria audience), here are a few articles about buffering:
KH, pH, and CO2
The CO2 content of your aquarium is important if you are growing plants (though check out a discussion on
which plants need supplemental CO2, and which may actually do worse with it!). But CO2 is also of interest to the general aquarist because it is intimately related to the water's carbonate hardness (KH) and pH; dissolved carbonates will raise both KH and pH, while addition of CO2 will lower pH. Here are some
charts showing the relationship between CO2, pH and KH, along with a discussion of the actual
chemical reactions. You can read CO2 content indirectly by from the combined pH and KH readings, from a good
test kit, or try this
simple metering method.
A few plants are able to take in carbon directly from dissolved calcium carbonate (a procedure called
biogenic decalcification by some). This procedure in turn may cause your pH to rise.
You can easily make your own
carbonate buffer from common chemicals. You can use
Calcium Chloride to increase GH without changing the alkalinity and pH. Some suggest you can use
hydrochloric acid in extreme moderation to reduce the buffering capacity of your water (this article also discusses an empirical result of CO2 content vs. KH).