H2O

Water
H2O, is the chemical symbol of water: one molecule of water is composed of two hydrogen atoms covalently bonded to a single oxygen atom. Water is a colourless, tasteless, and odourless liquid at ambient temperature and pressure. It is a very important solvent, capable of dissolving many other chemical substances, such as salts, sugars, acids, alkalis, some gases and many organic molecules.

Water is a chemical substance that is essential to all known forms of life. It covers 71% of Earth's surface. There are 1.4 billion cubic kilometres of it available on Earth. It appears mostly in the oceans (saltwater) and polar ice caps, but it is also present as clouds, rain water, rivers, freshwater aquifers, lakes, and sea ice.

Water in these bodies perpetually moves through a cycle of evaporation, precipitation, and runoff to the sea.


 * For a more detailed chemical description of water see Wikipedia.

Aquarium properties of water
Aquarium water can contain a great many chemicals which give it properties suitable for life to exist in it and it is this combination that gives water its characteristics that all aquatic life requires to live in, feed and reproduce in it.

Not forgetting that water at certain narrow band of temperature for particular species of animal will also support life. Too hot or too cold and that species will just die.

pH
The potential of hydrogen of the water is the measurement of the amount of hydrogen (H) and hydroxyl (OH-) ions dissolved in water. It has a logarithmic scale from 1 to 14, where 7 is neutral. less than 7 is acidic and more than 7 is alkaline.

Aquatic animals or plants can not live or breed outside their evolved pH band and a rapid change in pH can seriously injure or kill many types.

GH - General Hardness
General, sometimes called total or permanent hardness is a measure of the overall concentration of calcium, magnesium and other ions dissolved in the water.

Animals and plants become adjusted to the level of these ions and will not survive if the level of GH in the water exceeds their ability to adapt to it.

KH - Carbonate Hardness
This is the buffering capacity, temporary or carbonate hardness. KH is a measure of bicarbonate and carbonate ions in the water that act as buffers to prevent the pH changing too dramatically over a short time.

°C °F - Temperature of water
The temperature of water governs where or not life can exists in it. Too hot or too cold and the lifeform can not adapt and will die. Most fish do not generate their own body heat and therefore rely on the water temperature to provide the energy to metabolize their cells.

For example if the humble Neon tetra used to a temperature of is kept at  then all its body functions slow down and stop. It therefore can not breath and so the fish dies of oxygen starvation.

O2 - Oxygen
This chemical is vital for all life. However in water, the amount of oxygen that water can hold is determined by the temperature of the water (and the level of dissolved organic matter). As the water temperature drops so more oxygen can be dissolved in it. See the article Oxygen and Dissolved Oxygen (DO) levels for more details.

Tap Water
The usual source of water for the vast majority of aquarists. The quality of their tap water varies widely across the world or even from street to street. It can also vary over time.

Therefore it is important that the aquarist becomes familar with the of their particular tap water and monitor its properties over time.


 * Ask your tap water supplier for a recent detailed water quality report. This is usually provided free on request.

Pure water
In Nature there is no such thing as pure water. Pure water readily absorbs chemicals from its enviroment and will even absorb chemicals from most metal or plastic containers.

As all water contains bacteria, viruses, algae fungus and other microscopic lifeforms as well as chemicals. It is often required by aquarists to remove all of these to start from a known starting point. Perhaps to add chemicals and lifeforms of their own choosing.

There are several ways to make water clean and pure.

Distilled Water
This is water that is heated to boiling point and the steam is captured and collected. The result is water of a high degree of purity with no minerals or chemicals dissolved in it. This method is not used in the Aquarium hobby. Though small quantities can be purchased for medical use.


 * Its main benefit is that no bacteria or viruses exist in this type of water.

Reverse Osmosis (RO) Water
This is water that is passed against a membrane and because of a natural process called reverse osmosis, only the pure water passes through leaving the vast majority of the contaminates behind. It is not an efficient method however. Often 80% of the water passed against the membrane is wasted. Bacteria, etc. may pass through the membrane.

Many aquarists whose own tap water that is unsuitable for use in an aquarium or if they are raising aquatic animals who require clean soft water will use this method of making RO water.

Deionised Water
This is water that is force through filters, resin, active carbon, etc. to remove the chemicals and ions to leave the water virtually pure. In effect arriving at virtually the same product as distilled water. Often this water may have minor levels of bacteria, etc. picked up from the surrounding surfaces.

Aquarists often use these filters and resins to make their tap water much softer. But it can be expensive to run. However there is no water wastage.

Links

 * A gentle introduction to the stucture of Water
 * The Krib - What You Need to Know About Water Chemistry, and Why