Tropical freshwater aquarium fish reside in a wide range of water habitats from fast-flowing mountain streams, to great lakes, to slow-moving rainforest rivers. Each of these biotopes, besides having unique fish species, has unique water properties due to environmental factors. Because fish do come from unique natural water sources, their water in captivity should have similar characteristics. To recreate natural settings, the aquariast should have some knowledge of basic water chemistry, specifically pH, water hardness, and the nitrogen cycle.

The pH scale is a scale which is used to measure the acidity or alkalinity of a substance. The scale ranges from 0-14, with 0 being the most acidic, and 14 being the most alkaline. Water with a neutral pH of 7.0 has an equal ratio of H+ ions to OH- ions. Water with a pH below 7.0, is considered acidic (having a more H+ ions than OH- ions), while water with a pH above 7.0 is considered alkaline or basic (having a more OH- ions than H+ ions). Almost all freshwater fish inhabit waters with a pH from 5.0-9.0, with the majority of these inhabiting water with a slightly acidic to neutral pH (6-7.5).

In aquaria, the pH of tap water can be manipulated though the use of phosphate, bicarbonate, and other buffers. However the pH may change despite these buffers. The development of a low pH can usually be attributed to decaying material such as plant and animal wastes. When the pH drops below 5.5, bacteria (Nitrosomonas) that break down ammonium are reduced, and the ammonium level rises.
Lowering the pH: The pH can be lowered by using a pH-lowering chemical (available at pet shops). Be aware that many pH-lowering products use phosphate-based chemicals. Phosphate is a nutrient that encourages algae growth. The pH can also be lowered by plants and fish during respiration, when CO2 is exhaled into the water. In hard water, the pH may be difficult to lower. First, soften the water by peat filtration or reverse osmosis, and then try lowering the pH.
Raising the pH: The pH can be raised in an aquarium by adding baking soda (bicarbonate) or making a water change (thus removing some organic buildup which reduces pH).

Water Hardness

The degree of water hardness relates to the amount of dissolved minerals, especially calcium and magnesium, in the water. Water hardness is generally expressed in the amount of calcium carbonate (CaCO3). Water hardness is measured in ppm (part per million), kH (carbonate hardness), and dH (degrees of hardness) or gH (general hardness). Water is expressed as soft (having few dissolved minerals) or "hard" (having many dissolved minerals. General levels of water hardness are expressed in the table below (1 dH is equivalent to about 17 ppm).

very soft       0 to 70 ppm     0 to 4 GH (dH)
soft            70 to 135 ppm     4 to 8 GH (dH)
medium hard     135 to 200 ppm   8 to 12 GH(dH)
hard            200 to 350 ppm   12 to 20 GH (dH)
very hard       over 350 ppm        over 20 GH (dH)

For most aquariasts water hardness is not particularly important. However, excessively soft water can cause problems. Because softer water has less buffering capacity due to a lack of CaCO3 (a natural buffer), and is subject to pH crashes, where the pH falls dramatically causing harm to aquarium inhabitants.
Carbonate Hardness: Carbonate hardness (kH) is not used as a measurement of hardness as often. KH includes the presence of minerals and charged ions, other than Ca and Mg, dissolved in water.
Changing the Water Hardness: Water hardness can be manipulated in several ways. To make the water softer, the water can be filtered through peat moss or filtered through a reverse osmosis system. Ion exchange resins also can be used to lower the water hardness. Boiling water for a period of time can also reduce its hardness. To harden the water, filter the water through dolomite or crushed coral until the desired hardness is reached.
Electrical Conductivity: By running an electrical current through the water, the level of conductivity can be found. Conductivity indicates the amount of ions (electrically charged particles) are in the water. The higher the water hardness, the greater the conductivity. Testing the conductivity of the water only finds the total amount of ions present in the water, and does not give the origin of the ions, whether they are Mg, Ca, or Fe.

Aquatic plants, animals, and bacteria depend on oxygen dissolved in water for respiration. Oxygen can be added to the aquarium through an air pump attached to some sort of air stone, by utilizing a filter that creates much surface disturbance, or by using a wet-dry filter system. Plants also add oxygen during the day with photosynthesis, although use oxygen at night during respiration. Thus, in a planted tank, the oxygen levels fall at night. At a higher temperature, less oxygen is dissolved in water and more aeration is required.

Carbon Dioxide

Carbon dioxide is another gas present in water as a byproduct of the respiration of organisms in the aquarium. During photosynthesis, plants require carbon dioxide. When there is a lack of carbon dioxide, the leaves of plants begin to yellow, and growth slows. Some of the carbon dioxide dissolved in water forms carbonic acid, which lowers the pH. If there is too much carbon dioxide in the tank, the fish will suffer.
Adding Carbon Dioxide: Carbon dioxide can be added by lower the water hardness to free CO2 from calcium bicarbonate. CO2 can also be added by the use of a CO2 system.

The Nitrogen Cycle

When fishes excrete waste, plants deteriorate, and food rots, the resulting waste does not just disappear. The waste, in the form of nitrogen compounds, is broken down into other compounds by bacteria, until the final product, nitrate NO3 is absorbed by plants or removed through gravel siphoning. This process of converting NH3/NH4 (of fish, plant, and other waste) into NO3 is known as nitrification. The steps of this process are most evident when the tank is first established or when the tank is out of balance.

When the tank is first set up, there are not nitrifying bacteria present (unless gravel or filters from an already established tank are used). A few hardy fish are added to the tank. By the first day, trace amounts of ammonia/ammonium (more toxic ammonia if the pH is above 7.0, less toxic ammonium when the pH is below 7.0) have built up. Within a few days, the ammonia level rises to a toxic level. By this time Nitrosomonas bacteria have begun to develop. These bacteria begin to break down the ammonia into nitrite (NO2), which is less toxic than ammonia/ammonium. Another type of bacteria, Nitrobacters, break down the nitrite (NO2) into less toxic nitrate (NO3). The nitrate is absorbed by plants or algae, or is removed when a water change is carried out. Although nitrate is less toxic than other nitrogen compounds, in high levels, it can be toxic.

This cycle also occurs when the tank becomes out of balance. If waste levels become too high, because of over-feeding, overstocking, and/or lack of water changes, ammonia levels will rise again. In this case, water changes should be performed until normal levels are restored.

This cycle can occur when the bacteria that breakdown organic wastes die. This can occur when the tank or filter is washed with soap or some disinfectant. If this occurs, the tank will have to be cycled again. Be aware that too acidic (below 5.5 pH) water or some medications can also deplete the bacteria population.

     Nitrosomonas        Nitrobacters
NH3 -----------------> NO2 -----------------> NO3
excess food         absorbed by plants
fish waste      removed during water changes plant matter