Water acidity, active reaction of hydrogen ions, pH value.
Chemistry and biology in a fish aquarium are inextricably linked. A change in one factor always means corresponding changes in all other factors. The life of biological organisms in water is primarily determined by the active reaction of this water. As is known, a water molecule consists of two hydrogen atoms and one oxygen atom, its formula is written as H2O. Since there is always weak electricity in nature, under its influence a part of water molecules breaks down into hydrogen ions H+ and the hydroxyl group OH-. This process is called dissociation. Salts, acids, and alkalis dissolved in water partially dissociate into ions. pH means the content of H+ and OH- ions in the water formed from dissociation under the influence of electricity and is one of the most important indicators of water condition in the aquarium. When the content of H+ and OH- ions in water is equal, it is said that the water has a neutral reaction.
In such water, according to accepted international standards, 1 molecule out of 10,000,000 dissociates into ions. This figure can be expressed as 10-7. This is the active reaction indicator of water. 
It is customary to express this ion indicator in decimal logarithms, which implies a reverse exponent sign. The indicator of the active water reaction is given by the amount of hydrogen ions H+ and is called the hydrogen index pH. The pH scale is a straight line from 0 to 14, where the number seven means neutral water, this number is also called the hydrogen exponent. To the left of the hydrogen exponent are acidic waters (slightly acidic – acidic – strongly acidic), to the right – alkaline (slightly alkaline – alkaline – strongly alkaline). Since the pH value is a logarithmic quantity, a change in pH by 1 unit means a change in the concentration of hydrogen ions by 10 times, by 2 – by 100 times, and so on.
If during the reaction hydrogen is one of the products of dissociation, the water acquires acidic properties (acidic water reduces respiratory exchange); if the hydroxyl group, it acquires basic properties. Alkalinity is the total content of negative ions that neutralize hydrogen ions when acid is added to the water. Carbonate and bicarbonate ions neutralize increases and decreases of carbonic acid in the water, thereby ensuring pH stability. The processes of denitrification, photosynthesis, and sulfate reduction increase pH, while the prevailing processes of biological oxidation decrease it. A parameter such as water acidity has a great influence on biochemical and biological processes and is of great importance in the life of fishes. Ions in water are carriers of acidic or alkaline properties. Water acidity is designated by the symbol pH (an abbreviation for pondus Hydrogenii).
The expression pH < 7 indicates an acidic reaction, and pH > 7 an alkaline reaction. A neutral environment corresponds to pH = 7. The acidity of boiled tap water approaches pH = 7. Distilled water has a pH of 5.5–6.0, and not, as some think, pH = 7. In sea water, the indicators of this reaction are quite stable (pH 8.1–8.3), in fresh water they vary greatly depending on the season of the year and time of day, and they also differ in different layers of water. Life in water is possible within the range of pH 3.5–10.5. Environments with pH < 4.5 and pH > 9.5 are biologically aggressive and should be considered unsuitable for the life of aquarium inhabitants. Temperate zone waters usually yield an indicator between 7.5 and 9 — that is, they are slightly alkaline. Tropical waters, on the contrary, usually have a pH value from 5 to 6.8 — that is, more or less acidic. Of course, there are exceptions. The most famous of these are the Central African lakes. In Lake Tanganyika, this indicator averages 9.0 (in Lake Malawi located further south, it is about 8.4). And although the Zaire River is not so far from them, the water in it is somewhat acidic with a pH of 6.5.
Practically in most cases in aquariums, pH values fluctuate within 6.5–8.5. In long-uncleaned, polluted aquariums near the bottom, the pH can even be 5.4. For keeping most aquarium fishes, the water should be very slightly acidic, neutral, or very slightly alkaline (pH 5.5–7.5), and most plants feel normal at pH 6.0–8.0. For aquarium inhabitants, there are certain, so-called pH barriers, going beyond which, both to the left on the scale (towards the acidic side) and to the right (towards the alkaline side), is unacceptable. Moving aquarium inhabitants from one water to another when the difference in their pH values is greater than 0.8–1 is also unacceptable, as it can cause shock in fishes and rapid or gradual destruction of plant tissues.
If fishes find themselves in water with critical pH values (strongly alkaline or very acidic), they experience chemical burns of the gills, mucous membranes, and skin. Serious disorders of respiration and metabolism occur, and the body’s protective properties are significantly reduced. As a result, fishes can die from chemical burns or from other diseases that immediately affect fishes with weak immunity. It should be remembered that an increase in the carbon dioxide content in water leads to a shift in the hydrogen index towards the acidic side. That is, with weak aeration, especially after feeding, a large amount of carbon dioxide enters the water from under the gill covers of the fishes, which can in a short time turn the water from alkaline to acidic, which will certainly kill the fishes.
The hydrogen index of aquarium water has a significant impact on the vital activity of plants. With sharp changes in pH values, not only fishes but also plants can die. Plant death is usually observed when replacing old water with fresh water, as well as in spring and summer when aquarium lighting increases, especially from direct sunlight. More complex plants are difficult to keep precisely because their lower and upper pH barriers are close to each other — after all, in the flowing waters of their homeland, they do not experience such pH fluctuations as occur in aquariums with standing water.
In plant tissues, pH shifts lead to a slowdown in photosynthesis and other biochemical processes, and at critical values, tissue necrosis and chlorophyll breakdown are observed. It must be especially emphasized that fishes and plants can experience severe stress, chemical damage, and significant weakening even with very short-term fluctuations in the pH level in the aquarium. Therefore, before choosing fishes and plants for your aquarium, you must make sure that their requirements for aquarium water parameters, including pH, match.
The pH value in an aquarium is not stable; the pH indicator is very dynamic. The values of the active reaction of water just poured from the tap, settled water, and aquarium water tested in the morning and evening may differ from each other. Therefore, having once measured the pH value of tap water, one cannot assume that this value is established once and for all for your entire aquarium setup. The fact is that the pH value is influenced by such factors as the presence of free carbonic acid, which is a waste product of both fishes and plants, the dissociation of organic acids present in the water in one amount or another, water temperature, lighting, and the degree of water movement in the aquarium. In this regard, it can only be judged approximately. Trying to achieve absolute precision in the aquarium is pointless.
Let’s trace how pH changes in aquarium water throughout the day. During the respiration of hydrobionts, oxygen is absorbed, carbohydrates are oxidized, carbon dioxide is released, and 
energy is formed, which is used for vital activity. The entry of carbon dioxide into the water causes it to acidify. This means that all hydrobionts contribute to lowering the pH value through their respiration. This decrease is especially noticeable at night when plants do not absorb carbonic acid. During the day, during the light phase of photosynthesis, the activity of carbonic acid consumption by plants increases significantly. Carbohydrates and free oxygen are formed.
The absorption of CO2 by plants under good lighting can proceed so actively that the intake of carbonic acid exhaled by the same plants and other aquarium inhabitants does not compensate for the loss, which causes an increase in pH. This means that at night, the pH in the aquarium moves along the scale towards the acidic side, and during the day – towards the alkaline side. Only at night, when assimilation stops due to a lack of light, the plant in the process of dissimilation does not produce oxygen and does not need carbon dioxide, but exhales it, with the excess accumulating in bicarbonates.
However, the plant’s need for oxygen is so insignificant that fishes under normal conditions in a dormant state do not suffer at all. A negative impact can only be exerted by overpopulating the reservoir with fishes and an excess of plants without additional oxygen supply. Therefore, with prolonged bright lighting and a large number of plants intensively absorbing carbon dioxide, the pH value can rise to 9. And at night, due to the absence of photosynthesis and the release of CO2 by plants and fishes during respiration, it can drop to 6.
pH values change particularly sharply throughout the day across water layers if the water is not constantly mixed. In the upper layers during intensive plant photosynthesis, the pH can rise to 10–11, while near the bottom it will remain stable (say, around 6.6), and in the middle layers it will fluctuate from 6.5 (at night) to 7.0–8.0 (during the day). At pH 10–11, fishes descend to the lower layers, and the plants that caused such water alkalinization will begin to disintegrate in the near-surface layers.
In hard water, the water reaction is more alkaline, and acidity fluctuations are smaller than in soft water. Soft water dramatically changes acidity when a small amount of acid or alkali is added. The pH value in the aquarium is subject to fluctuations associated with changes in dissolved CO2 and the carbonate hardness of the water, KH. Thus, an increase in the amount of CO2 or a decrease in KH makes the water more acidic, while an increase in KH or a decrease in CO2 makes it more alkaline. Fluctuations in the pH indicator also depend on water temperature: as temperature rises, it decreases. For example, if measured at 0 °C, neutral water should be considered not at pH 7, but at pH 7.97 (almost 8). This means that water with pH 7 at 0 °C will already be slightly acidic. Fluctuations in pH are especially noticeable in soft water with negligible carbonate hardness.
Moreover, in actual practice, water alkalinization is extremely rare, whereas lowering the pH below the allowable level is very easy to achieve. Insufficient water aeration, an excess of nitrogen compounds and free organics in the water in the form of food residues and fish waste (if you have insufficient filtration and irregular water changes, as well as frequent overfeeding of fish), abuse of carbon dioxide fertilization for aquatic plants, etc. All these factors are certainly harmful in medium or high hardness water as well. But in soft water, they inevitably lead to acidification of the environment below acceptable limits and to the death of fishes.
For example, starting at a value of 7, after a month it may already be 5.8. A month later, during morning measurements in aquariums with abundant vegetation, values on the order of 5.0–5.5 can be obtained. In aquariums with a small number of plants or none at all, it can even be 4.7, and that around the clock. The water remains, as they say, “clean and transparent,” and most fishes remain alive (unless they are Malawi and Tanganyika cichlids and livebearing fishes).
To be more precise, let us add that the process of biogenic water alkalinization is also known. However, it is not observed very often, namely in aquariums with a very large number of aquatic plants under conditions of CO2 deficiency. The effect, once again, is threateningly manifested in soft water and is barely noticeable in hard and medium-hardness water. In general, calcium bicarbonate plays the role of a buffer in water, preventing sudden changes in acidity. This fact is a great blessing for us aquarists.
pH changes.
It is important to avoid sudden pH fluctuations. And what can be considered a sudden fluctuation? Answering this question, it must be borne in mind that essentially the pH indicator reflects the content of hydrogen ions in the water, and this indicator is logarithmic. Therefore, changing it by one unit corresponds to a 10-fold change in the concentration of these ions. Thus, a seemingly insignificant change – by just one unit – in the active reaction of water can cause stress in fishes. A rapid (a few tens of minutes) pH change of 2 units will highly likely lead to irreversible changes in the fishes’ gills and their gradual destruction. Having experienced such turmoil, fishes may die, and sometimes this does not happen immediately, but weeks later. Here, different sensitivities to pH changes affect both various fish species and individual specimens within a species.
To reduce sharp fluctuations, mandatory nighttime aeration of water with air that will carry away carbon dioxide is recommended. Oxygen is a universal oxidizer of organic compounds, and carbon dioxide is the foundation for the growth of underwater flora and one of the main regulators of the water’s hydrogen index – pH.
In addition to direct consequences, there can be indirect ones. When the pH level changes, some components dissolved in water become extremely toxic. For example, ammonia becomes even more poisonous in an alkaline environment. Did you know that even nitrifying bacteria, which break down ammonia into less toxic nitrites and nitrates, have their own pH limit at which they cope with their task and outside of which they stop coping? At a pH level of 7.5–8.6, these bacteria work well, but as soon as this level starts to fluctuate lower or higher, the bacteria fail. pH changes will also affect the effectiveness of treatment or the relative safety of fish when using medications during quarantine.
For example, chloramine-T becomes more toxic when the pH drops, while potassium permanganate, on the contrary, becomes very toxic in an alkaline environment. Monitor pH changes, and you will always be aware of what is happening in the aquarium, even if it is not yet visible. Changes in water composition will immediately let you know if, for example, your bottom filter is clogged. How will you know this? An increased content of dissolved waste in the water leads to the appearance of carbonic acid in the water, which in turn acidifies the water and lowers the pH level. Check the pH in your aquariums; values that are too low will indicate that the water in the aquariums is not being changed in the right amounts, or this is done too rarely. Of course, in a couple of weeks you will understand this anyway by the smell and color of the water, and then there will be far more problems.
pH – KH – CO2 equilibrium. Determination of the water’s active reaction indicator.
Aquarium water acidity. What to do when pH values change. 
