>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 
     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!