The influence of aquarium water hardness on aquatic organisms.
Water in tropical natural reservoirs is generally soft. This is because rivers and streams are fed by rainwater, which falls abundantly in equatorial regions. Rainwater is nothing more than water vapor condensed into droplets inside a cloud. Vapor, as is well known, contains no dissolved salts, and droplets passing through dusty air acquire a small percentage of hardness—usually no more than 0.2° dH. Upon falling to the Earth’s surface, rainwater dissolves salts and other substances present in the soil.
Since this process has been ongoing since the Earth’s formation, most salts have long been washed out of the soil, and water reaching rivers acquires a hardness of only about 2° dH. In many equatorial rainforest water bodies, this figure is even lower—practically 0° dH. The situation is different in rivers fed by underground springs. In this case, rainwater passing through the soil may encounter limestone and leach calcium compounds from it, increasing its hardness to 5-12° dH.
Water hardness has a major impact on the physiological condition of fish. Considering that hardness noticeably affects even the walls of iron pipes, it is easy to draw the following conclusion: animals and plants living in water all their lives are similarly affected. While an adult specimen may still survive in conditions different from its natural habitat, eggs, milt, and fry require strictly defined hardness parameters. Some fish species need soft water, others medium-hard or even hard water.
For keeping the most common fish species in aquariums, medium-hard water is quite suitable, despite the fact that the natural habitat of these specimens is very soft water bodies. This can be explained by the exceptionally developed ability of certain species to adapt, as well as the result of selection; over years of captive breeding, only specimens best adapted to tap water have survived from each generation. Today, these are true pets capable of existing in conditions that would be fatal to their wild ancestors.
It should be kept in mind that when transplanting fish from old water to fresh water and vice versa, they react sharply to changes in hardness—jerky movements, convulsions, jumping out of the water, and often death are observed. Generally, most fish and plants are able to successfully adapt to changing conditions, including unfamiliar water hardness (or softness). The ability to distinguish water parameters such as hardness and softness is necessary if you have to deal with some “difficult” fish species that have high environmental requirements.
As an example, we can cite special species of fish found in certain rivers of the Amazon basin, where the water is surprisingly soft and contains almost no minerals. If you keep unpretentious fish, it is usually sufficient to consider only water hardness. However, breeding fish in unsuitable hardness conditions is very difficult and sometimes completely impossible. For example, fish from the family *Characidae* react differently to this parameter. Thus, *Paracheirodon innesi* and flame tetras spawn in very soft water (0.5-4°), Hyphessobrycon heterorhabdus and serpae tetras in soft water (4-5°), while glowlight tetras and *Gymnocorymbus ternetzi* can breed in both soft and hard water. When choosing fish and plants for your aquarium, you must take their hardness requirements into account.
For example, it is completely unacceptable to house cichlids from the African lakes Malawi and Tanganyika, which need hard water, together with fish from the Amazon River basin, which need soft water. You need to decide in advance on your preferences and determine what water hardness will be in your aquarium and, accordingly, which fish will live there. If you put a fish whose ancestors lived in soft water for millions of years into hard aquarium water, it may survive for some time. However, calcium phosphate will accumulate in its renal tubules, forming crystals there, and the fish will weaken and die.
Conversely, if you place fish from hard-water regions—for example, from Lake Malawi—into soft water, their water-salt balance is disrupted, and ions are leached from the fish’s body into the aquarium water, where the salt concentration is very low. The fish weaken, their immune defense efficiency drops, and they perish. The fish itself will fight for its life to the end, straining all its ion channels, living with constant kidney pain. Do not torture animals just because you like them and want to admire them at home. In soft water, snail shells dissolve, and shrimp and crayfish have difficulty molting—these animals lack calcium.
In a multi-species aquarium, it is difficult to establish the water hardness required for each individual species. However, water hardness within 3-15° can be considered suitable for keeping most aquarium fish. Yet, deviations exist here as well. Livebearing fish require water with a hardness of 10-15°dH, characins prefer 3-6°dH, and Lake Malawi cichlids require 14-20°dH.
In Lake Tanganyika, three-quarters of the lake water’s salinity is due to the presence of carbonates. There are twice as many here as in Malawi, and three times as many as in Lake Victoria. For all African lake cichlids, dH 15-25°, KH 5-15° can be considered acceptable. Hard water lovers include: Malawians, Tanganyikans, Victorians, North American cichlids, as well as swordtails, mollies, and other poeciliids. Asian Etroplus and some Madagascar cichlids live in brackish water with an average hardness of 10-15°dH. Most South American cichlids inhabit the Amazon and Paraguay river basins, as well as the Guianas. Water there is usually soft, and its pH ranges from very acidic to alkaline.
Many species inhabit water bodies in northwestern South America, where the water is hard and more alkaline. Only these South American cichlids can be kept together with Central American ones, and even then not always. Rainbowfish from Australia prefer hard or medium-hard water with a slightly alkaline reaction. Labyrinth fish are best kept in soft or medium-hard water with a slightly acidic or neutral pH reaction. Experience shows that the presence of mollusks in an aquarium is not at all mandatory for establishing and maintaining a biological “balance”, but it is possible that in aquariums requiring soft water, mollusks are beneficial by converting calcium into an insoluble state.
Aquatic plants, which are quite sensitive to water hardness, prefer slightly hard water, although there are exceptions. For example, Madagascar *Aponogeton madagascariensis* and *Aponogeton boivinianus* grow in water with a hardness of 0.8-1.2°dH, but die in aquariums at a hardness of 4-5°dH. *Cryptocoryne ciliata*, conversely, grows at hardness levels exceeding 20-30°dH. 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. A decrease in the amount of carbonates in water leads to a drop in its hardness and is called biogenic water softening. The better illuminated the plants are in the aquarium, the higher this process is. Since general hardness depends on carbonate (temporary) hardness, plants cause its fluctuations throughout the day.
Under poor lighting, as well as at night, part of the CaCO3 salt returns to an ionic solution state. In soft water, CaCO3 reacts with carbon dioxide and significantly alters the pH value. Carbon dioxide dissolved in water actively interacts with water to form carbonic acid, which yields bicarbonate ions; these dissociate to give carbonate ions, and at all stages of this complex reaction, the water is enriched with hydrogen ions.
In hard water, calcium and magnesium act as a buffer, inhibiting these shifts. Therefore, in cities where tap water is soft and temporary or carbonate hardness is low, nocturnal fish kills and deaths of other pH-sensitive animals can occur in the aquarium. Often, *Cryptocoryne* species experience physiological shock and shed their leaves. Where water hardness exceeds 6°dH, such troubles can be avoided. For the same reason, *Cryptocoryne*, *Lagenandra*, and a number of *Aponogeton* species are best cultivated in water with a hardness of 6-8°dH.
It is known that the water in the Great African Lakes is slightly alkaline – pH 7.6-9.0. Similar conditions should ideally be created in the aquarium. However, aquatic plant reference books usually state that a pH of 7.5 is almost the upper limit of the active reaction for their normal growth. At higher pH values, it is very difficult to provide a sufficient level of carbon dioxide in the water required for the assimilation and growth of aquatic vegetation. According to this, it became clear that Malawian water is not very suitable for aquatic plants—does that mean we should acclimate the fish instead? Not at all.
Experience in growing aquatic plants in artesian water suggests that it is actually easier to acclimate plants to such a hydrochemical regime. In hard waters containing calcium compounds, plants release carbon dioxide from carbonate substances during the day. This process occurs as a complex chemical reaction, during which CaCO3 calcium salt is formed, precipitating out as acicular calcite crystals. This sediment covers the leaves of plants that obtain carbon dioxide in this way with a gray film—*Elodea*, pondweeds, *Cabomba* (not all aquarium aquatic plants possess this ability).
The influence of substrate, decorations, and shelters on water hardness.
Aquarium water hardness. Water hardness scale. 
