Aquarium Water Chemistry Basics

Appropriate water chemistry is of vital importance to the health of your aquarium fish.

Blue Streak Hap Cichlid (Labidochromis caeruleus). Via Xavier David /Wikipedia

Water is the natural living environment for fish, aquatic animals and plants; it is the essence of life for them, just as air is for us. Knowing the essential water parameters for the species you wish to keep, you can provide conditions similar to those found in nature. You do not have to be a chemist to master a few basic chemical terms. By understanding the chemical properties of water, you will be able to adjust its parameters at will. No matter what kind of water comes from the tap, you can modify it to keep the species of your choice.


Basic knowledge of water’s chemical parameters will help you achieve a better and more conscious maintenance of the animals and plants you already possess. Furthermore, it will come in handy in their reproduction, because the successful breeding of many species will require modification of these values. After all, as the creators and masters of this tiny world enclosed in a glass box, we have the responsibility to ensure the health and well being of the creatures in our care.

Water contains a lot of dissolved minerals that have major impact on its parameters; it is never “pure” unless it comes straight from a distiller or a reverse-osmosis (RO) filter—and then, stripped of its mineral content, it cannot sustain life.

Importance of pH in Aquariums

The first of the most important water parameters—if not the most important one—is the pH, defined as the measure of acids and bases in the water. You must have heard your aquarist friends mention that some species of fish, aquatic animals or plants live in acidic water, while others prefer alkaline. It was the pH that they were referring to.

This Endler’s livebearer prefers water that is hard to medium hard, with a neutral pH. Photo by Radek Bednarczuk.

To make measurements easier, the pH scale was invented, ranging from 0 to 14. A pH of 0-7 is acidic, a pH of 7 is neutral and pH of 7-14 is alkaline or basic. Test kits to measure the pH can be purchased in any aquarium shop. Usually, they contain test strips. Such a strip is simply dipped into the water to be tested, and it changes color, which is then compared to a chart included in the kit to find out the result. There are also professional (expensive) pH meters and liquid test kits. The latter are quite precise, so I recommend them. In the latter case, the testing process consists of drawing water from the aquarium into a test tube (or vial) and adding a few drops of a special reagent. Following a chemical reaction, the water will change color, which we then compare to a chart included with the kit to determine the pH value.

Fish can adapt to a pH that is outside the optimal range; however, a big difference—for example, when a species that inhabits acidic waters is kept in an alkaline environment—may negatively affect the fish’s health. It is important for the pH in the tank to be stable, without sudden swings. Any pH change of more than 0.5 in a day may cause a lot of stress for the fish.

Keeping some fish species in water that is too acidic will have dangerous consequences. Mucus will cover the surface of the gills, frequency of gill movements will increase and, from time to time, fin twitching can be observed; ultimately, deaths will occur due to poisoning with carbon dioxide, which accumulates in the blood. The dead fish will often have tightly closed gill covers and mouths.

If a fish is quickly transferred, such as from a bag, into a tank with much more acidic water (a sudden pH drop), it will make sudden jumping/darting movements, develop spasms and ultimately perish. On the other hand, a pH that is too high for acidic-water fish will result in restlessness, increased respiration, mucus appearing on the body, fins fraying and the eyes becoming cloudy.

In both cases—unless the gills have become irretrievably damaged—the affected fish should be put in water with a neutral pH to regenerate, which can take up to two months.

General Hardness: dGH

Some fish, such as African cichlids from Lake Tanganyika, live in hard water, and others, such as those that inhabit the Amazon basin, live in soft water. We can tell hard water from soft in a simple way. Hardness is mainly due to the presence of two elements: calcium and magnesium. If your tap water is soft, which is common in mountainous areas, a small amount of soap is enough to wash your hands and produce a lather. Hard water, on the other hand, will mean that more soap will be necessary to produce a lather. When tap water is hard, calcium deposits will form in the plumbing and appliances.

Midas cichlid
The Midas cichlid adapts easily to a variety of water conditions. Photo by Radek Bednarczuk.

Rainwater, distilled water and water from the RO filter are naturally soft. Popular test kits for measuring general hardness use German degrees, on a scale extending from 0 to 30 dGH (degrees of general hardness), from very soft to very hard. The testing process itself is similar to pH testing: We take water from the aquarium and add a reagent, drop by drop. By counting the number of drops necessary to cause a color change, we find the general hardness value. In the United States, more than 85 percent of households have hard water (Wilson, Amber; Parrott, Kathleen; Ross, Blake. “Household Water Quality —Water Hardness.” June 1999).

There is one more chemical term relating to water hardness—carbonate hardness, also known as carbonate alkalinity.

Carbonate Hardness/Carbonate Alkalinity: dKH

Degrees of carbonate hardness (dKH) is known as temporary hardness—it “disappears” during boiling with the formation of calcium and magnesium carbonate (limescale) deposits, as it is the hydrogen carbonates of these elements that are largely responsible for it. The dKH, too, is usually measured in German degrees.

Celestichthys margaritatus
The colorful Celestichthys margaritatus likes water hard and alkaline. Photo by Radek Bednarczuk.

You should know that the dKH is inseparably linked to the pH, so it is advisable to keep the former at > 5 to avoid sudden pH swings. As a rule, the higher the dKH, the more difficult it is to change the pH. With a high enough dKH, the pH should remain stable for a long time. If there is some difficulty in maintaining a stable pH, consider raising the dKH.

Other Measurements of Interest

Aside from dGH, dKH and pH, which are the most important parameters in freshwater aquarium hobby, there are other chemical measures of interest to the aquarist. In a marine tank, salinity should be tested. There is the also the level of carbon dioxide and iron (useful largely for owners of aquatic plants). Phosphate tests can be helpful in the event of a sudden algae outbreak—the more phosphates in the water, the more algae in the tank. To decrease the concentration of phosphates, make a large water change and increase the number of plants.

There are also tests for oxygen concentration; they are typically used in the case of overstocking, very high temperatures or poor fish health. In densely planted aquaria, the levels of this gas can drop at night, when plants cease to produce oxygen but keep absorbing it. Oxygen is also used up in the aquarium during the decomposition of organic substances, such as dead plants, food remains or fish waste.

The Aquarium Nitrogen Cycle

The name might sound daunting for the beginner, but the whole concept is not so difficult to master. Fish and other aquatic animals (shrimp, snails, turtles etc.) excrete metabolic waste directly into the water in which they live. These substances are broken down by beneficial bacteria, known as nitrifiers. These helpful but invisible inhabitants of our aquaria colonize naturally; the more of them there are, the smaller the amount of harmful compounds in the tank.

Plants are very good at absorbing metabolites; the more plants there are, the less algae and the better living conditions for the fish. Photo by Radek Bednarczuk.

To jump-start the cycle in your aquarium, ready-made bacterial cultures can be bought at aquarium shops. Once added to the tank, they quickly proliferate and spread. To grow, they need food, such as fish excrement, and a lot of oxygen. They will also multiply quicker at a temperature above 68 degrees Fahrenheit. The optimum pH for nitrification ranges from 6 to 8.

These bacteria prefer to grow on various surfaces, and they mostly settle in the filters (especially on the porous material, such as lava, foam, etc.), as well as on the substrate and decorations. The relationship is simple: The more porous the material, the more bacteria can grow on it. If while cleaning an old filter you see jelly-like gunk/slime covering the media, that’s the colonies of useful bacteria.

Whenever a fish excretes waste, you get ammonia, which is a toxic substance. Ammonia is converted by the aforementioned bacteria to less-dangerous compounds called nitrites, and then to even less-dangerous compounds called nitrates, which are the end product of the cycle. The presence of beneficial bacteria shows that the biological filter is functioning properly.

At least 14 to 42 days must pass before the full nitrogen cycle—ammonia to nitrite to nitrate—develops in a newly set up aquarium, and that’s assuming that the temperature is 68 degrees or higher. It will take even longer at lower temperatures.

In a mature (cycled) aquarium, the levels of ammonia and nitrites should be zero (undetectable with aquarium test kits), while the level of nitrates steadily climbs. To reduce nitrates, you need regular (weekly or bi-weekly) water changes. We can make our job easier by introducing some plants, such as Echinodorus ranunculoides, that uptake nitrates. A denitrifying filter, which removes nitrates, might also be used.

In pet shops, you can buy test kits for all these three chemical markers.

The first—ammonia—should not be detectable at all. It is at its most toxic in alkaline water; above pH 8, it will, among other things, destroy the delicate gills of fish.

If poisoned with the second—nitrites—our fish will lose the coloration characteristic for the species, and in the presence of high concentrations of nitrites, will simply perish. Some species, such as flame tetra, Harlequin rasbora or Badis badis, are particularly sensitive to these compounds.

Water Parameters and the Death of Your Fish

  1. A sudden alteration in the water’s chemical parameters may negatively affect the health of your fish and even kill them, especially when pH changes.
  2. Any modification of the water’s chemical parameters must be done on the basis of tests; making changes “blind” will harm the animals in your care.
  3. If you don’t want to test the water yourself, go to the nearest aquarium shop; the people there will tell you the exact parameters of the water in your tap.
  4. If you don’t feel up to changing water parameters, simply match the fish you buy to the kind of water you have.
  5. If unexplained fish deaths occur, first check for the presence of ammonia and nitrites in your tank. You should also see if there are any anaerobic zones in the substrate (gray or black areas), which when disturbed emit bubbles of stinky gas (smelling of old eggs). Finally, test the pH. Only after ruling out all of these factors can you look for a disease that is killing your fish.

Test kits for ammonia and nitrites are necessary at the start of a new aquarium, but will also come in handy later to monitor conditions. Your filter could fail mechanically, or your fish could require medication that may destroy nitrifying bacteria. You will need to test the water regularly during these events and adjust the water to avoid fish deaths.

The end product of the nitrogen cycle—nitrates—can also be harmful in high concentrations, but mainly for fish fry, whose growth they can slow down or even stunt. In adults, high nitrates could cause reproductive problems and even infertility. This is why nitrate levels should be checked at least once a month.

Aquarium Fish Populations and Fish Waste

The relationship is simple: The more fish in the tank and the more frequent the feedings, the more waste is produced. Add this to the fact that the fewer the number of plants and the less frequent the water changes, the higher the nitrate concentration will be in your tank. Therefore, a small number of fish, fed sparingly and provided with lush plant growth and frequent water changes, will experience lower nitrate levels.

Some fish species, like this Acarichthys heckelii, thrive in soft acidic water. Photo by Radek Bednarczuk.

It is important to remember that a sufficient concentration of bacteria depends, among other things, on good oxygenation of the aquarium water. Turning off the filter for the night to conserve power, or experiencing a power failure, in addition to the use of chlorinated water, UV radiation and certain chemicals, such as the popular methylene blue and other medications, may destroy, or at least reduce, the population of these useful microorganisms, threatening the whole cycle.

Furthermore, in the absence of oxygen, toxic hydrogen sulfide can be produced in the filter (look for the odor of rotten eggs). This gas can also be generated in the substrate as a result of decay of organic matter—such as fish waste, food remains, etc.—if the substrate is not oxygen-permeable (as it would be in fine sand, for example). In such a case, dark regions in the substrate can be observed when viewing the aquarium from the side, and when the substrate is disturbed, bubbles will rise; you’re sure to smell the egg odor. A thorough removal of any remaining dark patches on the bottom and changing the substrate for something more coarse (and more permeable to oxygen) should help. It is worthwhile to know that hydrogen sulfide, even in small concentrations, can suffocate the fish.

Playing Chemist

Once you know the main chemical parameters and how to check them with aquarium test kits, you might want to alter some of them to make them more suitable for the fish. Match the fish you buy to the water in your tap, and all your troubles are over. However, if you wish to keep species that come from hard water and you have soft water in your tap, or the other way round, you need to modify it.

To make soft water hard: You can buy ready-made reagents at an aquarium shop and proceed according to instructions. Alternatively, you can add baking soda (2 teaspoons for each 27 gallons, on the average) and you will raise the electrical conductivity and slightly elevate the pH.

Electrical conductivity (conductance G) is influenced by the presence of ions (cations and anions) resulting from dissociation of dissolved salts and ammonia and carbon dioxide. The electrical conductivity indicates the degree of mineralization of water (with salinity). The water conductivity is thus linked to its hardness. The harder the water, the higher the electrical conductivity.

Add limestone, coral gravel, ground-up shells of sea snails or Roman snails, etc., to the tank as a natural way of making the water harder.

To make hard water soft: You can buy special water-softening products, such as ion-exchange resins, which work by removing the calcium and magnesium ions and replacing them with sodium. You can also use peat, which will both soften and acidify the water. However, the most spectacular effect can be achieved by using distilled water or buying a reverse osmosis filter. The water thus obtained can then be mixed in the correct proportions with tap water to achieve the desired hardness. You can also enrich RO water with special mineral salts available in shops to make it similar, for example, to the waters of South America.

How to Change the pH

In order to lower the pH value slightly, it is best to choose natural peat products, either in liquid or in solid form (granular or fibrous). Some people also employ ketapang leaves, oak bark and leaves, or dry alder cones. A safe and effective way to lower the pH is by adding CO2 (carbon dioxide) to the aquarium. CO2 dissociates in the water and some of it forms carbonic acid, which reduces the pH. For this method to work, a constant supply of carbon dioxide is necessary (for instance, from a pressurized CO2 system) to ensure pH stability, because when CO2 fertilization is turned off, the pH will return to its original value. The high cost of such systems, however, makes them impractical as a method for lowering the pH in most aquaria.

Aquarium Water Testing

Beginners should test for ammonia and nitrites, as well as pH and dKH (the latter parameter is inseparably linked with the former). The first two tests will help you to find out whether the aquarium is fully cycled. Furthermore, these test kits will be helpful whenever you need to determine the cause of fish deaths (which usually happen to beginners).

If you don’t feel confident fiddling with water chemistry, simply match the fish, other animals and plants to the water you have in your tap.

Radek Bednarczuk has kept fish since he was 7 years old, when he received his first aquarium with viviparous fish (a couple of swordtails). A few years later, he started keeping cichlids and has been fascinated by them ever since. He works as a pharmacist. He has written nearly 200 articles, published internationally, on the subject of fishkeeping, and is a regular within Aquarium Live, a German fishkeeping monthly. He has also written two books on fishkeeping. Visit his website at

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