Aquarium water hardness.
Water is one of the most unusual chemical compounds on Earth with complex properties. Aquarists need to know, be able to determine, and regulate some of them because the health and life of aquarium inhabitants depend on temperature, active reaction, hardness, salinity, concentration of dissolved gases, and metabolic waste products. Water contains substances that give it properties such as color, transparency, odor, as well as dH hardness values and pH hydrogen index.
Hardness is one of the most important parameters of freshwater, which determines the possibility of keeping and breeding fish and cultivating plants in it. 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. Water hardness is a property of natural water that depends mainly on the presence of dissolved calcium and magnesium salts. Furthermore, it does not matter in what chemical compounds these substances are.
The amount of calcium and magnesium depends on the type of surrounding soils and ground, catchment area, season, weather, and time of day; their dH decreases or increases. Naturally, water taken from water bodies in different regions of the world differs significantly in hardness. For example, passing through limestone, water becomes mineralized and hard. Conversely, seeping through peat, water softens – its general hardness decreases. With an insignificant amount of these elements, water is called soft; with a large amount, hard. With the change or elimination of temporary hardness, the total hardness of the water also decreases. Consequently, the hardness indicator is as unstable as other water parameters.
The results of hardness determination are usually expressed in German degrees dH and KH. Under natural conditions, calcium, magnesium, and other alkaline earth metal ions that cause hardness enter the water as a result of the interaction of dissolved carbon dioxide with carbonate minerals and other processes of rock dissolution and chemical weathering. Water hardness varies within wide limits. Large fluctuations in temporary and total hardness can negatively affect the health of aquarium inhabitants. Calcium hardness usually predominates (up to 70%); however, in some cases, magnesium hardness can reach 50-60%. In Lake Tanganyika, magnesium hardness predominates, which is why breeders sometimes practice adding magnesium sulfate to the water.
In an established aquarium, water gradually softens because calcium is absorbed by fish, invertebrates, and plants. Therefore, old water tends to be soft. In a heavily planted aquarium and under strong lighting, hardness can change throughout the day. During the day it decreases, and at night it increases. This is especially dangerous for aquariums where green algae have developed. Depending on its properties, water promotes or inhibits vital processes. Clean, transparent tap water containing all the trace elements necessary for plants, with dH 5-20°, KH 2-15°, and pH 6.5-7.5, is suitable for the aquarium.
Total hardness is measured in dH – this value shows the total amount of alkaline earth metal ions in water, such as calcium, magnesium, strontium, beryllium, and barium. The physiological significance of these elements for freshwater fish is extremely great, but they consume these elements not from the water, but from food. Strictly speaking, in the vast majority of cases, this parameter is completely unimportant to fish. Only extreme values are dangerous for them.
Thus, in water with a dH value close to zero, nothing living will survive at all. And at very high values exceeding 35-50°dH, other factors come into play; here freshwater aquascaping ends, and the realm of natural laws for brackish waters begins. Various osmotic phenomena start to take effect, and far from every freshwater fish will survive under such conditions. In all other cases, most fish easily adapt to any reasonable total hardness values.
Please note: dH, KH, and pH form the Bermuda Triangle of water chemistry. Although these three parameters are different, they all interact with each other to varying degrees, making it difficult to change one parameter without changing the others. This is one of the reasons why beginner aquarists are advised not to mess with water parameters unless absolutely necessary. For example, hard water often occurs due to limestone. Limestone contains calcium carbonate, which when dissolved in water increases both dH (from calcium) and KH (from carbonate). Increasing KH also usually increases pH. As a result, KH acts like a sponge, absorbing acid from the water and increasing pH.
Hardness is an important parameter for the aquarist, but it is worth noting that it is determined only by the amount of certain dissolved minerals. There are other substances that contribute to the total mineral content but do not affect water hardness and can be measured using special tests. Water containing a small amount of such salts or none at all is called soft. Based on the same criterion, although mineral-free water (i.e., with minimal mineral content) is soft, such water does not necessarily have to be completely devoid of minerals. It is very important to remember this, as some water softening methods do not reduce the mineral content, but simply convert the salts that make water hard into other salts that do not affect hardness.
In general, the hardness of surface waters is usually lower than the hardness of groundwater. Surface water hardness is subject to noticeable seasonal fluctuations, usually reaching its highest value at the end of winter and its lowest during the flood period when it is heavily diluted with soft rain and melted snow. Sea and ocean water have very high hardness. Water hardness varies within wide limits, and there are many types of water classifications based on the degree of hardness.
Water hardness scale.
Very soft —— 0 – 4° up to 1.5 mg eq/L — 0 – 70 ppm
Soft —— 5 – 8° — 1.5 – 4 mg eq/L — 70 – 140 ppm
Medium hardness — 9 – 12° — 4 – 8 mg eq/L — 140 – 210 ppm
Hard —— 13 – 22° — 8 – 12 mg eq/L — 210 – 320 ppm
Very hard —— 23 – 34° above 12 mg eq/L — 320 – 530 ppm
Water hardness is measured in German degrees. Municipal services measure water hardness in milligram equivalents of calcium and magnesium ions per liter of water (mg eq/L). One German degree corresponds to 0.36 mg eq/L. To convert to German degrees, you need to divide the water hardness in mg eq/L by 0.36. Thus, a hardness of 5 mg eq/L equals 14 German degrees (5 : 0.36 = 14). Conversion of water hardness from German degrees to degrees of other countries:
a) 1 American degree must be multiplied by 1.04.
b) 1 English degree must be multiplied by 1.25.
c) 1 French degree must be multiplied by 1.79.
Water hardness has two different measurements. The term dH means hardness degrees, while ppm stands for parts per million, which is roughly equivalent to mg/L. 1 degree dH equals 17.8 ppm CaCO3. Most test kits show hardness in degrees CaCO3, and this means that the hardness is equivalent to the amount of CaCO3 in the water, but it does not mean that this exact amount of CaCO3 is actually contained in the water.
Keep in mind that when people say a fish “prefers hard (soft) water,” they mean total hardness (dH) (and not KH). Do not confuse it with carbonate temporary hardness KH. It should be noted that dH does not affect the pH level of water, although “hard” water tends to be more alkaline because total hardness is related to carbonate hardness KH. Inappropriate water hardness negatively affects the absorption of nutrients through cell membranes and the excretion of metabolic wastes, can affect egg fertilization, the functioning of internal organs such as kidneys, and animal growth.
Before setting up an aquarium, it is advisable to find out at least the approximate hardness and active reaction of the tap water, and then consult a specialist: they will suggest which fish and plants will suit you. Every aquarist should know at least the hardness of their home tap water. And how do you know if you are pouring suitable water into the aquarium for your pets? Specialized stores offer supplies, reagents, and devices with which you can determine water properties at home. Water should not contain a large amount of calcium and magnesium salts. Hardness can range within 4-12°.
Very hard tap water needs to be softened. Water hardness test kits can be purchased at pet stores. They can be used both to check tap water and aquarium water (for example, to determine whether aquarium decorations affect water hardness). In most cases, people can tell whether their tap water is hard or soft by the effects it produces at home. Hard water requires more soap to produce lather than soft water, and moreover, hard water causes scale to form on the inner surfaces of plumbing fixtures and metal kettles. This scale is mainly calcium carbonate (CaCO3).
Despite the “public opinion” prevailing in your area regarding basic water parameters, at the first opportunity try to determine everything yourself, or try to get information from the local water treatment plant. Tap water sometimes has a hardness above 8 degrees dH. Such water is not suitable for creating soft-water biosystems and needs correction. Soaked wood very often helps with this. Driftwood placed in the aquarium shifts the pH slightly, but sometimes this is enough.
When selecting fish for an aquarium, you must first divide them into groups that require similar maintenance conditions. These conditions are: temperature, hardness, and pH values of the water. Aquarium practice has established that most freshwater fish have significant adaptability, i.e., they get used to changing water composition very well. This property is necessary in nature because water quality can vary. For this purpose, we divide temperature and hardness values into three groups:
• Temperature: 1). Lower than 22°C; 2). 22-26°C; 3). Higher than 26°C.
• Hardness: 1). Softer than 8°; 2). 8-12°; 3). Harder than 12°.
• pH values can simply be divided into two groups: 1). Acidic water (< 7). 2). Alkaline water (> 7).
If you want to stock your aquarium with certain species of fish, first make such a table and see if their living conditions match each other. You can find the table parameter values for a specific fish species from a good aquarium book. A beginner aquarists should not rush to fill their aquarium to the limit. By acquiring fish slowly and gradually, it is easier to decide which fish is best for your aquarium. Do not try to combine the uncombinable, for example, keeping cold-water and warm-water fish in the same aquarium, or those that require different water compositions: some need “old”, acidic, soft water, while others need regularly changed, fresh, alkaline, hard water.
The effect of aquarium water hardness on hydrobionts.
General, Carbonate, and Temporary water hardness. 
