For that reason, distilled water rarely has the theoretically pure pH of 7.0. Deionized water is obtained by filtering it through ion-exchange materials that remove cations (sodium, calcium, magnesium, potassium, ammonium) and anions (chloride, sulfate, bicarbonate, nitrate). Deionization results in very pure water, but it retains non-ionized contaminants such as organics, carbon dioxide, oxygen, nitrogen, chlorine, silica. Water for deionization is usually pretreated with activated carbon or organic scavenging resins to remove organics and some gases. Either distillation or deionization produces water suitable for most of the more demanding water needs of hobbyists. Such water usually has a pH of about 6.0, due to dissolved carbon dioxide. Reverse osmosis water is obtained by filtering a slow moving stream of water under high pressure against a fast moving stream under relatively reduced pressure. The purity of R/O water is variable, depending on the source water and the performance characteristics of the R/O unit. Some sophisticated laboratory systems can produce water that approaches either distillation or deionization, but, due to the inability to maintain adequate pressures, most home or hobby units produce only marginally improved water over the source water, even when operated conservatively. Many hobbyist units are not realistically rated.

Is it necessary to use distilled or R/O water in an aquarium or is tap water good enough?

The use of tap water, once chlorine and chloramines have been removed should prove satisfactory for most hobbyists. Sometimes, tap water may contain excessive phosphates or nitrates or both for use in reef tanks or with more delicate juvenile freshwater fish. Soft acid water may also be contaminated with copper extracted from pipes. In such situations, processed water may be a good choice. If you wish to set up a soft water environment for Discus or Tetras, and your tap water is very alkaline and hard, processed water may be necessary. If processed water is used in a freshwater aquarium it may be advisable to replace some of the essential salts that have been removed. The most practical, reliable, and economical way to do this is to use a good salt water mix, 1 teaspoon per 10 gallons is about right. You don't need to spend money on a costly special product to add minor elements to processed water. In the case of R/O water, it is unlikely that the process was efficient enough to seriously deplete minor elements from the water in the first place.

How do natural water environments differ?

Native water environments are freshwater, spanning a full spectrum from very soft to very hard, from acidic to alkaline. By comparison, marine water, although it varies somewhat, is relatively consistent. Estuaries generally represent brackish waters, variable mixtures of fresh and marine waters. Soft water has a low calcium and magnesium content and such water usually has an acid pH with low acidity and low alkalinity. It may, however, have a high pH or a high acidity, but high alkalinity is very rare in soft waters. Hard water, in contrast, has elevated calcium and magnesium concentrations, usually with high pH and high alkalinity, although high acidity may also be possible, but low pH is unusual. It is a mistake to assume that, if water has a high pH, it must be hard water.

What is hardness, alkalinity, conductivity, specific gravity?

Soft water, by definition, is water which contains very little dissolved divalent cations, specifically, calcium and magnesium, whereas hard water contains high concentrations of these. Water that contains less than 35 mg/L calcium/magnesium (equivalent to about 90 mg/L calcium carbonate or 5 German degrees) is generally considered soft, whereas water containing more than 175 mg/L calcium (equivalent to 446 mg/L calcium carbonate or 25 German degrees) is considered hard. Despite the much confused state of misinformation prevalent in the hobby, hardness has absolutely nothing to do with carbonates. The terms “permanent” and “general” hardness and “carbonate” hardness are obsolete and should be discarded. “Permanent” or “general” hardness is true hardness; "carbonate" hardness is not hardness at all but alkalinity. Conductivity is another parameter that confuses the picture even more. A solution of sodium chloride has high conductivity, but no hardness or alkalinity. A concentrated solution of calcium chloride is very hard, but has no carbonate or alkalinity, but has high conductivity. A concentrated solution of sodium bicarbonate or sodium diphosphate has a very high alkalinity, but no hardness, but high conductivity. Hardness, alkalinity, and acidity contribute to conductivity, but conductivity is not any of them. Conductivity is a measure of the water's ability to conduct an electrical current and reflects the concentration of dissolved ions. Hardness is assessed by measuring calcium and magnesium. Historically, hardness was an assessment of water’s ability to precipitate soap, a property directly dependent on the calcium and magnesium concentrations. The problems arise with test kits and conventions that report hardness as some unit of calcium carbonate. Worse, some so-called hardness test kits actually measure alkalinity. The consequence of this is that many a hobbyist confuses hardness with alkalinity, a measure of the water's ability to resist a drop in pH, calcium carbonate or calcium oxide. To avert this problem, manufacturers and hobbyists should avoid the usage of units that utilize calcium carbonate or calcium oxide as a point of reference. Forget grains or mg of calcium carbonate! Forget German degrees or KH! Hardness should be expressed for what it is, a concentration of divalent metal ions as mg/L. Likewise, alkalinity should be expressed for what it is, the ability to resist change in pH on the measured addition of acid, meq/L. An often overlooked parameter is acidity, which is a measure of the water's ability to resist change in pH on the addition of base. Likewise, acidity should be expressed for what it is, the ability to resist change in pH on the measured addition of base, meq/L. Taken together, acidity and alkalinity constitute the buffer capacity of the water, the ability to resist change in pH from either direction. The assessment of buffer capacity should be as important as the measurement of pH, since rapid change in pH poses a greater hazard than does pH itself. Conductivity is of questionable usefulness: hard water will always have high conductivity, but high conductivity does not necessarily mean hard water; high alkalinity will always give high conductivity, but, again, high conductivity does not necessarily mean high alkalinity. Conductivity tells you how much dissolved ions are in the water, but does not tell you anything about what kind they are. Any dissolved substance that ionizes will raise conductivity: sodium, calcium, chloride, sulfate, even tannic acid.