There has been an increasing interest in water chemistry in recent years and the aquarist is faced with a growing number of products and processes which promise to keep aquarium water perfect. It is little wonder most of us become confused. The basics of water quality seem to be tremendously complex.
No one should keep plants and fish together without learning a little about water chemistry, but some hobbyists exaggerate the complexities of water chemistry. If you learn the few essential, tried and proven rules of fish-keeping, you can easily learn enough water chemistry to be successful.
Basics should include a little knowledge about water hardness, the carbonate system, pH values and the nitrogen cycle.

Water Quality & Total Hardness

All spring and river water contains calcium and magnesium in varying quantities. The most important elements are calcium bicarbonate and calcium sulfate. Water rich in calcium salts is considered "hard": with little it is called "soft". Hardness is measured in "degrees of hardness", one degree being equal to 10 mg of calcium or magnesium oxide per liter of water.
Hardness caused by calcium bicarbonate is called temporary or transient hardness since it disappears when water is boiled. The hardness resulting from calcium sulfate is considered "permanent" since it remains. Together temporary and permanent hardness produce overall or "total hardness". Another term you'll hear is "carbonate hardness", explained in the next section.

dGH = dKH + PH (Sulfate)
GH = Total Hardness (dGH = German Total Hardness)
KH = Carbonate Hardness (dKH = German Carbonate Hardness) PH = Permanent Hardness

This equation cannot be used with commercial hardness test kits. When measuring Total Hardness the cations, that is a positively charged ion, Ca²⁺ and Mg²⁺ are measured. While measuring the carbonate hardness the anions of HCO₃^- are determined. The carbonate hardness may be greater than the total hardness because of the presence of sodium, potassium and other cations in addition to calcium and magnesium. They do not cause hardness but may occur together with the bicarbonate anion to increase the quantity of bicarbonate. The following table shows the three basic ways of determining total and carbonate hardness.

Total hardness has a direct effect on the cellular functions of fish, plants and microorganisms. The most favorable values for aquatic life are between three and ten degrees, although these may be too low for Tanganyika- and Malawi cichlids.
Total hardness can be illustrated as follows:

DEGREES OF HARDNESS
0 - 4° dGH = very soft
4 - 8° dGH = soft
8 - 12° dGH - medium hard
12 - 18° dGH = fairly hard
18 - 30° dGH = hard
over 30° dGH = very hard

Carbon Dioxide and Plants

Carbon dioxide (CO₂) dissolves readily in water and forms small quantities of carbonic acid. The salts of carbonic acid and simple carbonates usually account for the largest part of the electrolytes in aquarium water. This means the absorption of carbon dioxide by plants is closely linked to the complex system of aqueous carbonic acid and carbonates.

The Carbonate System

Carbon dioxide is about forty times more readily soluble in water than oxygen but diffuses some ten thousand times less readily from water than air. About 0.2% of the dissolved CO₂ is converted to carbonic acid (H₂CO₃). As you add CO₂ the quantity of carbonic acid increases and the pH drops. As CO₂ is removed the pH rises. Carbonic acid disassociates or decomposes gradually, in two stages:
1st stage: H⁺ + HCO₃^-
2nd stage: H⁺ + CO₃^2-

Two important salts needed in an aquarium are calcium bicarbonate [Ca(HCO₃)₂] and calcium carbonate (CaCO₃). Calcium bicarbonate dissolves easily in water and produces temporary or carbonate hardness. This evaporates or disappears when the water is boiled while calcium carbonate is practically insoluble in water and is largely deposited. One example: the deposits on the wall of a tea kettle. Calcium bicarbonate continues to circulate in an aquarium only if an amount of carbon dioxide is in solution. The quantity of CO₂ is called the "equilibrium CO₂" and when insufficient causes some of the calcium bicarbonate to decompose into barely soluble carbonate.
There is a relationship between the carbon dioxide content, the pH value and the bicarbonate when one considers equilibrium and balance. One outward sign: various forms in which carbon dioxide combines (CO₂ + H₂C0₃, HCO₃^-, CO₃^2-) display a characteristic distribution dependent on the pH value.

• In acid water (pH of less than 6.0) free carbon dioxide is it
  solution and carbonates are negligible. (Example: Calcium deficiency in peaty water.)
• In neutral and slightly alkaline water (pH 7.0-8.0) most of the 002
  is found as bicarbonates. (Example: normal aquarium water.)
• In highly alkaline water (pH over 10) most of the CO₂ exists as
  carbonate. Dissolved CO₂ is generally absent above pH 9.0.
  (Example: The sodium-rich water in which certain cichlids live.)

The carbonate system is a mixture of a weak acid (carbonic acid) and its salts and, in a chemical sense, is a typical buffer. Buffers have the special property of combining with moderate additions of acids or bases so that the pH value barely changes. When an acid is added to a buffer the H⁺ in the bicarbonate is bound, producing carbonic acid which breaks down into carbon dioxide and water with the residue showing little tendency to dissociate. The H⁺ concentration is increased only slightly and the pH value remains constant.
If a base is added to a buffer, the OH^- ions are immediately fixed by CO₂ and bicarbonate is formed. The loss of carbon dioxide has only a slight effect on the carbonic acid content (H⁺-ions concentration) and the pH value rises very little.

As you add acid it diminishes since bicarbonate is not present below pH 6.0. As you add bases it also diminishes since carbon dioxide cannot be found beyond pH 10. The quality of the buffer depends on the water's mineral carbonate content. The buffer action of hard water is much better than of soft, peaty water and the more hydrogen carbonate aquarium water contains, the more free H⁺-ions which can be caught up. This is known as the acid-combatting ability of carbonate.
There is a close interplay between the carbonate system or carbonate hardness and pH values. In most cases, the higher the hardness, the higher the pH value but at the same time the pH value is buffered or stabilized better. A carbonate hardness between 2° and 8° is recommended.

pH Values

pH assigns values to the degree of acidity in water and indicates a change from a chemically neutral point. Neutral water H₂O, contains equal parts of hydrogen ions (H⁺) and hydroxide ions (OH^-). Hydrogen ions make water more acid and hydroxide ions make it alkaline. A pH of 7.0 indicates neutral water. Values above 7.0 indicate increasingly alkaline water while those below indicate increasingly acidic water.
Changes in concentrations of H⁺ and OH^- can be measured in grams. pH 7.0 means that 10^-7 of a gram of H⁺ is dissolved in one liter of water. A pH of 3.0 indicates 10^-3 grams. A pH of 10.0 indicates 10^-10 grams.

Since negative powers are cumbersome the pH scale is used. It runs from 1 to 14 as follows:

pH	1	2	3	4	5	6	7	8	9	10	11	12	13	14
H+-ion concentration (g/l)	10-1	10-2	10-3	10-4	10-5	10-6	10-7	10-8	10-9	10-10	10-11	10-12	10-13	10-14

It is also worth noting that a change of one unit in pH equals a ten times change in the acidity or alkalinity of the water. Two units indicate a 100 times change, three units 1000 times and so on. The optimum pH for aquarium fishes lies between a pH 5.0 and pH 9.0. Most freshwater fishes prefer a pH between 6.0 and 7.5 while most marine fish prefer values between 8.0 and 8.5.

The pH value is closely associated with bicarbonate anions and the buffering effect of the carbonate system. Make regular pH checks. All inhabitants of your aquarium, from fish to microorganisms, are sensitive to changes in pH. Abrupt fluctuations, often as a result of carbonate disturbances, can lead to serious problems with your fishes, acidosis, alkalosis and others.

Nitrification

Nitrogen is one of the vital elements of protein and is absorbed by green plants in the form of nitrate. Along with ammonia both occur in small quantities in most natural waters. The situation differs in an aquarium where plants, fish and other animals are raised together. Nitrogen compounds can be created quickly by feces, urine and other excreta, plant remains and decaying food. When concentrated, the compounds can have a harmful effect on the tank's inhabitants. An aquarist must set up and maintain his tank as free as possible of harmful nitrogen compounds.
Organic nitrogenous substances decay in stages in the presence of oxygen, a process called oxidative breakdown. It produces various nitrogen compounds as follows:

Organic nitrogen compounds —> Ammonia and Ammonium Nitrite —> Nitrate.

Toxic ammonia and non-toxic ammonium are produced in the first stage of the nitrogen cycle. The pH value greatly determines which of the two will predominate. Ammonia occurs at a pH of 7.0 and over: ammonium at a pH of less than 7.0. Ammonia build-up cannot occur in water which has the slightest acidity and from this one can understand the importance of regular pH checks.
Few plants utilize ammonium, nitrate being the chemical most can use.
The second stage of the nitrogen cycle is nitrite which is the result of the bacterial oxidation — some call it the 'combustion'-- of ammonia or ammonium. The bacteria which promote nitrification belong to Nitrosomonas sp. Nitrite is also very toxic and harmful to fish. Guppies (Poecilia reticulata) are susceptible to 1 mg/I and the threshold is even lower for other fish. The threshold of toxic ammonia is higher than that of nitrite.
In the third and final stage nitrite is converted to nitrate. This is promoted by the Nitrobacter bacteria and is much less toxic. Nitrate becomes harmful only in very high concentrations. Those above 150 mg/I should be avoided since denitrification, that is the reduction of nitrate into nitrite and ammonia, increases.
The nitrogen cycle cannot occur without oxygen. If the supply in an aquarium is poor, organic compounds decay more slowly and the water is enriched with interim toxic products — ammonia and nitrite. Bacterial denitrification can also occur. Some experts suspect the process can even occur within the bodies of the fish. Nitrite accumulates in the red corpuscles of fishes' blood and hinders the absorption and transportation of oxygen.
Nitrosomonas and Nitrobacter are not immediately present in a new aquarium and must develop and multiply over a period of time. The bacteria are primarily found in filter and substrata gravel elements of an established aquarium and can be transferred from one aquarium to another. It may take several weeks (up to 100 days) for them to reach the numbers where they can break down organic nitrogen compounds in a new tank. In the interim the quantity of bacteria in an aquarium will fluctuate widely.

Each water and filter change reduces the amount of available bacteria and affects the tank's equilibrium. For this reason we do not recommend changing the two simultaneously. Allow at least
one week between changes.

Solving water problems

Reducing hardness

• Make a water change using distilled or clean water. Never use rain water.
• Use special equipment to soften water. A commercial water softener or an ion exchanger are suggested.
• Filter water through peat.
• Use a reverse osmosis unit.

Increasing hardness

• Carefully add calcium or magnesium sulfate.
• Make a water change with hard water.
• Filter water through marble chips or coral sand.

Lowering the pH (acidulation)

• Filter water through peat.
• Make a partial water change.

Increasing the pH (alkalinity)

• Add sodium bicarbonate (NaHCO₃) or sodium carbonate (Na₂CO₃)
• Aerate water vigorously to expel CO₂.
• Make a partial water change.
  

Reducing Nitrite and Nitrate

• Regular water changes are suggested. (Intervals differ with the species of fish.)
• Add a biological filter — an add-on tank filter, a corner sponge filter etc.
• Add filtering material from an established tank. Gravel, filter media, etc. contain important nitrifying bacteria.
• Regular cleaning of the filtering system.
• Reduce feeding. Increases in nitrite and nitrate can often be traced to over-feeding.
• Add live plants to the tank.
• Clean the tank, removing dead fish, decaying plants, stems etc.