Like all the topics in this website we discuss water hardness in a series of discussions. The simplest explanation is first, followed by a more in depth discussion. Then there are links to very in depth discussions of both GH and KH hardness.
The Simple Explanation
There are two types of hardness use in the aquarium hobby. General hardness (GH) and carbonate hardness (KH). GH and KH most often move in lockstep but it is possible to have high GH and no KH and no GH and high KH.
GH (“general hardness” or “total hardness”) is just the amount of calcium and magnesium in the water. This is the white crap that builds up at the waterline of aquariums. GH is unimportant for ALL adult fish and almost all eggs and fry.
KH (“carbonate hardness” or “alkalinity”) is the amount of “buffering”. Buffering in this case reflects the ability of the water to resist downward drops in pH. KH is unimportant for ALL fish, adult or fry in any aquarium with decent water changes.
It cannot be emphasized enough:
KH and GH are unimportant in aquariums!
Just keep the pH between 6.5 and 8.5 and the total dissolved solids above 50 (i.e. all water except distilled and RO) and all fish do fine. GH and pH only become important in breeding a few species of fish and for shrimp and snails. KH becomes important with intensive cycling of aquariums.
Hardness in More Depth
The terminology surrounding the two types of hardness is very confusing since different authors use different terms for each type of hardness. This terminology is best illustrated with a chart:
For our purposes we will simply use four terms for hardness: Carbonate Hardness or KH and General Hardness or GH.
The term use of the term “alkalinity” to describe carbonate hardness (KH) is the source of a lot of confusion. Alkalinity is the buffering capacity of a water body; a measure of the ability of the water body to neutralize acids and bases and thus maintain a fairly stable pH level. This buffering is generally provided by carbonates and bicarbonates.
Carbonate hardness (KH) refers to the amount of the anions (negatively charged ions) carbonate and bicarbonate in the water. Bicarbonate and carbonate neutralize acids by producing carbon dioxide gas which gasses off from the aquarium. An aquarium with high KH will resist something called “old tank syndrome”, where the water in the aquarium becomes very acid.
Very acid water stops the “cycle” in the tank and is bad. When one is intensively cycling an aquarium with water on the soft side one needs to constantly add calcium carbonate to keep the KH up above 18 KH.
General Hardness (GH) refers to the amount of the cations (positively charge ions) calcium and magnesium in the water. Calcium and magnesium carbonate salts tend to be very insoluble in water and precipitate out readily, forming the white line at the waterline in older aquariums. Calcium and magnesium cannot be removed from an aquarium directly. But calcium and magnesium can be removed by pretreating the water with either reverse osmosis systems or water softening systems.
GH can be the same, higher or lower to the KH depending on the cations and anions in the sample. For example, a large amount of NaHCO3 (sodium bicarbonate or baking soda) would raise the KH and not affect the GH. A large amount of MgSO4 (magnesium sulfate or Epsom Salts) would raise the GH and not the KH.
If one overlays the maps of KH and GH in the tap waters in the USA they DO NOT correlate well. There are large areas with tap water which is high in KH (like 400) and low in GH (like 20). The range of GH is 10 to 250 while the KH is 0 to 400.
Much like pH, aquarium water hardness, both GH and KH, just is not very important. The published hardness ranges for the native waters of most aquarium fish are quite broad. And all fish can survive just fine well outside those ranges. Snails and shrimp require some GH.
Because general hardness and carbonate hardness are not actually as closely related to each other as the non-chemist is prone to believe, these two topics are best discussed separately. There are some aquarium hobbyists who are interested in delving deep into the science and the calculations behind all aspects of the hobby. And some hobbyists like to see all the research as to why hardness is not very important in the aquarium. For those who are so inclined the following more in-depth articles are pertinent:
4.5.1. General Hardness (links to aquariumscience.org)
4.5.2. Carbonate Hardness (links to aquariumscience.org)