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Chloride
>While I'm at it, it would be interesting to hear data about Cl-, and
>any biological effects of excessive chloride ion content on the
>inhabitants of freshwater tanks.
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 is
the dominant anion in seawater, where it is present in concentrations
of 19,300 ppm. (Seawater has a total dissolved salt content of 36,000
ppm, where freshwater generally has a total salt content of less than
500 ppm). Chloride salts are usually very soluble in water, and are
nonreactive with most other chemical species.
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. Fish maintain a relatively constant
internal salt concentration, under normal conditions. Freshwater and
marine fish have very similar chloride concentrations in their
tissues.
Of course, every anion is accompanied by a cation, so a high chloride
concentration corresponds to a high salt concentration. So, a high
chloride "problem" is actually a high salinity problem. If your
aquarium has a high chloride content, then you either have a marine
tank or it's time to do a water change! :-)
BTW, some freshwater fish have an extremely high tolerance to high
salinity--goldfish and carp can tolerate salinities as high as 17,000
ppm, while some killifish (Fundulus) can live in both freshwater and
seawater. The capability of freshwater fish to adjust to abnormally
high concentrations of nontoxic salts depends on species-specific
factors such as the gill-to-body surface ratio, gill histology,
hormonal control of membrane permeability, and oxygen and temperature
levels. Plus, the slower the salinity change, the more tolerance a
fish has. I suppose you could slowly add salt to your discus tank
over a period of several days, and end up with a rather salty discus
tank with no ill effect on the fish.
I'll bet Hach or LaMotte could develop a hobbyist grade chloride test
kit and make a fortune selling to nervous aquarists!