4.5.2. Carbonate Hardness

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Author : David Bogert

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Carbonate Hardness

“Carbonate hardness” or “KH” is a confusing term because it does not refer directly to what most people associate with the term “hardness”. “Hard water” is normally thought of as high GH water which can be so “hard” it creates “hard” deposits in water lines. These “hard” deposits what the term “hardness” is normally associated with. High KH alone will not produce “hard” deposits in a pipe.

The pH, KH and carbon dioxide relationship is the “Bermuda triangle” of fish keeping: go there and you may never come out. What is important is that KH is just not important in the aquarium. This fact is sufficient for most hobbyists. If one is a real nerd (like the author) and wants to become very confused, read on:

Lethrinops sp. gold Harbour Island
Lethrinops sp. gold Harbour Island.

KH refers solely to the buffering alkalinity (the ability of a solution to resist a pH change with an addition of an acid) from the carbonates and bicarbonates in the water. KH refers to only the bicarbonate, and carbonate anions (negatively charged ions); it does not measure the sulfates and other anions. KH comes from the German term “Karbonatharte” or carbonate hardness.

The term “alkalinity” is commonly used to indicate KH. Note that the terms “alkaline” and “alkalinity” refer to different things. “Alkaline” refers to a pH greater than 7 while “alkalinity” refers to the buffering capacity of the water. To confuse matters even more many use the terms “total alkalinity” to refer to the KH when in actuality “total alkalinity” measures both hydroxides and carbonates together.

Julidochromis transcriptus Kalemie
Julidochromis transcriptus – Kalemie

The Bermuda Triangle of Fish Keeping

CO₂, pH and carbonates are supposedly related by the following three equations:

CO₂ + H₂0 = H₂CO₃ (Carbonic Acid at low pH)

H₂CO₃ = H+ + HCO₃ (Bicarbonate at moderate pHs)

H+ + HCO₃= 2H+ + CO₃(Carbonate at higher pHs)

In truth, this isn’t the way it works. Carbonic acid doesn’t exist. And the existence of the carbonate and bicarbonate are dependent on certain cations like sodium and calcium to produce a high pH that allows them to exist. This very complex relationship is analyzed in depth in these two articles:

4.4.3. Carbon Dioxide and pH

15.6.2. CO₂, KH and pH in a Planted Aquarium

In addition the other anions and the cations in the water all affect this triangle to one degree or another by affecting the pH which in turn affects the KH. Complicating the triangle even more the whole thing is very affected by the temperature of the water. Bicarbonate converts to carbonate at higher temperatures. And the whole thing is affected by the variable of time. Some of the chemical reactions occur in seconds and some take days.

photo of an aquarium fish angelfish

If one has a planted tank, things get even more confusing. All plants and algae take carbon dioxide out of the water and produce oxygen. Some plants and algae take bicarbonate and carbonate out of the water and produce oxygen. So the KH of the water in a well planted, well lit tank CAN possibly fall to close to absolute zero after several hours of being in the light.

But complicating this fact is that carbon dioxide from the atmosphere CAN take the carbonate back. And plants excrete carbon dioxide in the dark. There are no studies on the effect of KH on fish simply because of that high variability.  If you measure your KH understand that the KH may well be far different in just a few hours. KH goes up and down constantly in most aquariums. If this all confuses you, join 99.9% of the population.

The author has raised and bred Lake Malawi cichlids, Lake Tanganyikan cichlids, guppies, mollies and platies in the soft acid water of Connecticut. The author has also raised and bred blackwater rams in the hard alkaline waters of Florida. Oscars, a fish native to the very soft, acid, low KH waters of the Rio Negro river (in all fairness some authorities dispute that), have naturalized in the high dKH waters of Florida and are an invasive species there.

Benthochromis horii
Benthochromis horii

A reading of zero KH is no problem at all. It will not create health problems with the fish. Many like to keep a higher KH in order to prevent a phenomenon called “Old Tank Syndrome”. Old Tank Syndrome is actually rarely seen if you are doing any sort of decent water changes. But adding KH with something like calcium carbonate (crushed coral or aragonite sand) to the aquarium will not hurt anything.

Carbonate is important for cycling. KH or carbonate is consumed by ammonia as it converts to nitrate. This can drop the pH down and stop the beneficial bacteria from working. So one normally has to add baking soda during intensive cycling. Each 10 ppm of ammonia converted to nitrate consumes about 72 KH (4 dKH). Or one gram of most ammonium salts consumes about two grams of sodium carbonate or three grams of sodium bicarbonate. One never wants zero KH during cycling. This will stop the cycling cold.

My tanks all have a bag of crushed coral in the filter at all times. Note that some get concerned with a high KH. A high KH per se is no problem. Also note that most tap water in the USA is high KH (15 to 25 dKH, 300 to 450 KH).

Pygocentrus nattereri Red Piranha
Pygocentrus nattereri Red Piranha

KH and Ammonia

Many say that if the pH swing from a low pH to a high pH the relatively harmless ammonium in the water will be converted to harmful ammonia and the fish will suffer. By this mechanism supposedly KH becomes very important. This is only true in an aquarium with very poor filtration and very high ammonia.

Virtually all the aquarium ammonia tests measure the TOTAL “ammonia”, i.e. ammonium plus ammonia. And in any aquarium with decent filtration the “ammonia” test should register 0.25 or lower. So any healthy aquarium should not have ammonium to be converted to ammonia at a high pH.

OB Peacock
OB Peacock

KH Terminology

KH terminology is confusing because different people use different terms for it, often incorrectly. Here are some of the terms used:

Synonyms for “Carbonate Hardness”
ppm CO₃, HCO₃
dKH (KH/17.9)
ppm CaCO₃
Amatitlania nigrofasciata Polar Blue Convict
Amatitlania nigrofasciata Polar Blue Convict

Further Analysis

The complex relationship between KH, CO₂ and pH is, as we said above, the “Bermuda Triangle” of Fishkeeping. But if one really is a nerd like the author and really wants to understand it one has to do a LOT of reading. These articles all examine various aspects of this triangle.

CO₂ and pH in the Aquarium

The complex relationship between CO₂ and pH is explored in depth in this article:

4.4.3. Carbon Dioxide and pH

KH and Cycling

KH is important in intensive cycling of an aquarium. This topic is covered in this article: KH and Cycling in an Aquarium

Planted Tanks and KH

If one wants to understand the relationship between carbon dioxide and KH in a planted tank these articles will be of some benefit:

15.6. Carbon Dioxide in the Planted Tank

15.6.2. CO₂, KH and pH in the Planted Tank

GH or General Hardness

If one wants to look at the science behind the other type of hardness, namely GH or general hardness, go to this link:

4.5.1. General Hardness.

Red Flame Peacock
Red Flame Peacock