General, Carbonate, and Temporary Water Hardness.
Water is a universal solvent on our planet; all life depends on water and largely consists of it. Water dissolves, in one way or another, all existing chemical elements, including gases. The possibility of fish and plants surviving in water strongly depends on its composition—not only on what it contains, but also in what proportions. Freshwaters vary greatly in hardness. Fresh or salt water from natural water bodies contains a certain amount of calcium ions as well as magnesium ions.
The content of calcium and magnesium in water determines water hardness. Calcium and magnesium are among the most active regulators of chemical processes in nature, and hardness is perhaps the most important indicator of water quality, regardless of the purpose for which it is used—for drinking, heating, or keeping fish. Depending on the season, weather, soils, and ground through which sedimentary and underground waters pass, their gH decreases or increases.
Naturally, water taken from water bodies in different regions of the world varies significantly in hardness. For example, passing through limestone, water becomes mineralized and hard. Conversely, seeping through peat, water softens—its general hardness decreases. When these contents are low, water is called soft; when present in large quantities, it is called hard. In addition to this division, calcium and magnesium components are also distinguished. These elements are very close in properties, although slightly different. Thus, the solubility of magnesium salts is usually lower, and their excess imparts a bitter taste to the water.
The following types of hardness are distinguished:
General hardness. Determined by the total concentration of calcium and magnesium ions. It is the sum of carbonate (temporary) and non-carbonate (permanent) hardness.
Carbonate (temporary, bicarbonate) hardness. Caused by the presence of calcium and magnesium bicarbonates and carbonates (at pH > 8.3) in water. This type of hardness is almost completely eliminated by boiling water and is therefore called temporary hardness. When water is heated, bicarbonates decompose to form carbonic acid and precipitate calcium carbonate and magnesium hydroxide.
Non-carbonate (permanent) hardness. Caused by the presence of calcium and magnesium salts of strong acids (sulfuric, nitric, hydrochloric) and is not eliminated by boiling (permanent hardness).
Carbonate hardness consists of calcium and magnesium bicarbonates that decompose upon boiling. During boiling, bicarbonates decompose, forming a salt precipitate, and carbonate hardness decreases. Therefore, carbonate hardness is also called temporary hardness. Carbonate hardness is generally lower than general hardness. The hardness remaining in water after boiling is called permanent hardness—it is formed by substances such as sulfates, chlorides, nitrates, silicates, and phosphates, and its value cannot be reduced by boiling the water.
General hardness is the sum of temporary and permanent hardness, the sum of all dissolved alkaline earth metal ions. With the change or elimination of temporary hardness, the general hardness of water also decreases. Consequently, the hardness indicator is as inconstant as other water indicators. Water hardness fluctuates especially sharply during algal blooms (“water blooms”). Large fluctuations in temporary and permanent hardness can adversely affect the health of aquarium inhabitants. In an established aquarium, water gradually softens because calcium is assimilated by fish and plants. Therefore, old water tends to be soft. In an overgrown aquarium and under strong lighting, hardness can change throughout the day. During the day it will decrease, and at night it will increase. This is especially dangerous for aquariums where green algae have developed.
General hardness is measured in gH (General Hardness)—this value shows the total amount of alkaline earth metal ions in the water, such as calcium, magnesium, strontium, beryllium, and barium. The physiological significance of these elements for freshwater fish is extremely high, but they consume these elements not from the water, but from their food. Strictly speaking, in the overwhelming majority of cases, this parameter is completely unimportant to fish. Only extreme values are dangerous to them. Thus, in water with a gH value close to zero, nothing living will survive at all. And at very high values exceeding 35-50° GH, other factors come into play; here, freshwater aquarium science ends, and the realm of natural laws for brackish waters begins.
Various osmotic phenomena begin to take effect here, and far from every freshwater fish will survive under such conditions. In all other cases, most fish easily adapt to any reasonable values of general hardness. For keeping any fish, in the vast majority of cases, general hardness should not interest aquarists at all. Except perhaps just once, to make sure your water does not have extreme deviations.
pH and kH are interconnected, and the relationship between these parameters is very simple. The higher the bicarbonate content in the water, i.e., the kH value, the higher, and most importantly, more stable the pH value, as it serves as an excellent water buffer. Pay attention to the word “stable.” In this context, it means that water with a high carbonate hardness of over 12 degrees has an alkaline reaction. Water of medium hardness (6-12 degrees) will likely be close to neutral (pH ~7.0), while water with a carbonate hardness of less than 6 degrees kH can, broadly speaking, be anything, but will most likely be acidic. Lower values increase the likelihood of an acidic crash.
Every aquarist should know at least the water hardness of their home tap water. How else can they know if they are pouring suitable water into the aquarium for their pets? In aquarium literature, “German degrees of hardness” are most often used when indicating hardness, designated as dH (less commonly dGH – deutsche Gesamthärte). Specialized stores offer supplies, reagents, and devices with which you can determine water properties at home. Water should not contain large amounts of calcium and magnesium salts. Hardness can fluctuate within 4-12°. Very hard tap water needs to be softened.
Generally speaking, most fish and plants are able to successfully adapt to changing conditions, including unusual hardness (or, conversely, softness) of water. The ability to distinguish between water parameters such as hardness and softness is necessary if you have to deal with some “difficult” species of fish that have high environmental requirements. However, breeding fish under inappropriate hardness conditions is very difficult and sometimes completely impossible. When choosing fish and plants for your aquarium, you must take their water hardness requirements into account.
For example, it is completely unacceptable to house cichlids from the African lakes Malawi and Tanganyika, which require hard water, together with fish from the Amazon River basin, which require soft water. You need to determine your preferences in advance and decide what water hardness will be in your aquarium and, accordingly, what fish will live there.
Aquatic plants, which are quite sensitive to water hardness, prefer slightly hard water, although there are exceptions. For instance, the Madagascar Aponogeton species *Aponogeton madagascariensis* and *Aponogeton boivinianus* grow in waters with a hardness of 0.8-1.2° dH, whereas in aquariums they die at a hardness of 4-5°. Cryptocoryne ciliata, on the contrary, grows at hardness exceeding 20-30°. Since plant development in very soft water is simply impossible, the question of this water type does not arise for most aquarists, regardless of whether it can be replicated or not. In soft water, snail shells dissolve, and shrimp and crayfish have difficulty molting—these animals lack calcium.
Most aquarium fish and plants in their native habitats live in very soft or soft water. In our taps, medium-hard or hard water generally flows. But do not despair—thanks to the fact that fish and plants have lived in aquariums for years, they have adapted to increased hardness. This applies to “undemanding” fish and plants; however, if you want to keep soft-water-demanding living organisms, you will have to soften the water. Most aquarium fish kept in aquariums live normally at 3-15° hardness. But here too we encounter exceptions. Livebearing fish need water with a hardness of 10-15° dH, Characidae prefer 3-6° dH, and Lake Malawi cichlids require 14-20° dH. Some gobies from Asian rivers die very quickly in soft water.
Aquarium water hardness. Water hardness scale.
Methods for increasing or decreasing pH. 
