Electrolytes.

Salts, which dissociate into ions when they go into solution, will conduct electricity in water; that's why they are electrolytes. In water, rock salt (NaCl), for example, dissociates into a sodium cation and a chloride anion. The sodium and chloride ions are electrolytes because they affect the conductivity of electricity. Chemically pure H₂0 is a very poor conductor of electricity. As electrolytes are added to distilled water, it begins to conduct electricity. In this way, you can measure the electrical conductivity of water, if you have a special electronic meter, and get a useful picture of its level of dissolved salts.

The major inorganic solutes ordinarily found in water are "cations" (ions with a positive charge) of Sodium (Na), Potassium (K) Calcium (Ca) and Magnesium (Mg) and "anions," (ions with a negative charge) of Chloride (Cl), Sulfate (SO₄), Phosphate (PO₄), and Carbonate (CO₃).

Thus salt dissociates in water into two electrolytes. In the early generations of fish-keeping, before World War II, traditional aquarists maintained a low level of common salt in freshwater aquaria as a "tonic," and to "prevent" disease. Even today a few aquarists still habitually add salt to freshwater on an "it couldn't hurt" basis. Frankly, salt has no business in a freshwater aquarium, unless you are keeping fishes that come from a brackish environment: mollies, for example. Since I haven't kept any brackish water fishes--— except mollies, when I was a kid--— I can't tell you anything about brackish aquaria.

Still, salt is a useful temporary medication. Potassium chloride (KCl) might be just as beneficial. On the subject of the chloride anion in freshwater, Mark Fisher posted to the Aquatic-Plants Digest, 22 Jan 1997, a reassuring post. If you don't follow up links, the gist of it is: "I would love to know why so many people seem to think chloride is a problem-- it is not! Chloride is a stable, relatively nonreactive anion that is ubiquitous in ground and surface waters... Chloride occurs in the blood and tissues of all living organisms (plants, humans, and aquatic organisms alike). The chloride concentration in fish blood is approximately 8,000-11,000 ppm, depending on the species."

He goes on to say "Of course, every anion is accompanied by a cation, so a high chloride concentration corresponds to a high salt concentration." If you were substituting potassium chloride, that would be a K cation instead of a Na cation. All the beneficial "tonic" effects of NaCl-- countering nitrite toxicity is the only one that comes to mind, frankly-- are connected with the chloride anion.

Recent experiments are showing that environmental cues for gonad maturation are connected to decrease in electrical conductivity rather than to low pH values themselves (though the two are linked), and to increases in water depth, all tokens of a rainy season. Dr. Frank Kirschbaum, of the Leibnitz-Institut, Berlin, experimented with the gymnotiform Rhamphichthys, some mormyrids and African schilbeid catfishes, etc. and reported to the IX (1997) meeting of the Neotropical Ichthyological Association, who posted an abstract of his paper, "New aspects concerning the cyclical reproduction of tropical freshwater fishes" (scroll down at that site for it). Amateur fishkeepers became aware of this role of reduced conductivity, with increased water depth and lowered temperatures, when Dr. Kirschbaum published a bombshell article in Heiko Bleher's quarterly magazine Aqua Geographia no. 20 (2000) under the title "The breeding of tropical freshwater fishes through experimental variation of exogenous parameters."

Within a year, hip Germans cued to aquatic newsgroups like de.rec.tiere.aquaristik were refering to the "Kirschbaum method" for unlocking natural hormonal spawning keys for the kinds of fishes that are currently only forcibly spawned in Southeast Asia through hormone injections.

Measuring the conductivity of water. The conductivity of water is measured in units called Siemens per meter. In the aquarium you'd use microSiemens per centimeter. This requires a special handheld probe tester that measures a minute amount of electricity passing between probes that are a centimeter apart. I've never actually handled one of these myself. But if you're using r/o water and want to know just how much of your tapwater to mix with it in order to match the water in the aquarium, or if you want to test whether your D/I membrane or resins are still effective, well, you should know your way around conductivity testing. A good way to dive into this area is with a www.google.com search: "test conductivity aquarium water microSiemens."

"Electrolytes" and osmoregulation. Fish can't survive for long in pure distilled water. Even fishes from the softest waters of the upper Amazon require some electrolytes for osmotic regulation ("osmoregulation"). Osmosis refers to the controlled diffusion of molecules through a semi-permeable membrane. Osmoregulation keeps a cell from draining its watery contents into more concentrated salt water and collapsing like a raisin. Or, conversely, osmoregulation keeps a cell bathed in freshwater from taking in water and blowing up till it bursts. The membrane most concerned in fishes' osmoregulation is the outer covering of the gills. Organs like the fishes' kidney system and the gills are also concerned with osmoregulation on a larger scale, which in a sense is the sum of many cells operating together.

Hence the electrolytes offered in water "conditioners" are quite fairly said to be "essential." However, the only time you ever might need to add electrolytes is if you were using pure distilled water and nothing but. The presence of unspecified "essential electrolytes" in water conditioners is generally a code term for common salt (NaCl), a harmless contaminant that is expensive to eliminate. The traces of sodium and chloride essential for any freshwater fish scarcely need to be added to any natural water. It is potassium that's actually required, not sodium. "Potassium ions always serve as the chief intracellular electrolytes, even though sodium ions predominate in the environment," writes Franklin M. Harold in The Way of the Cell (p 56). Calcium and magnesium are other electrolytes that are more desirable than sodium.

When you are offered the chance to buy concentrated electrolytes in a bottle, remember, the Skeptical Aquarist says "the cheapest source of electrolytes usually is a splash of your own hard tapwater."