09-16-2012, 02:12 PM
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It is difficult to pinpoint precise levels of GH and pH at which health issues will absolutely occur. Scientific studies to attain this would have to be very extensive and take into account various factors; not all fish are exactly the same to begin with. By the way, KH is immaterial, as this has no effect on fish or plants, though it does of course serve to buffer pH.
There is also the issue of wild caught fish versus tank raised. The latter have been common in parts of Europe for more than a decade now, and only recently in North America. Observations to date would suggest that tank-raised fish have fewer problems with slightly harder water, though spawning apparently does not occur. But I must emphasize that this is what i have read, and may or may not be complete. So i will confine my remaining comments primarily to wild-caught fish. And, the cited data comes from an article in TFH in 1987 by Rolf Geisler and Sergio Annibal, Ecology of the Cardinal Tetra, Parachierodon axelrodi (Pisces, Characoidea), in the River Basin of the Rio Negro, Brazil, as well as Breeding-related Factors.
Starting with the levels in their habitat, the GH is so low it is basically non-existent. GH of 0.03 dGH was the highest sampled. The pH varied from 3.97 to a maximum of 5.1. More recent samplings by eminent sources such as Heiko Bleher do not vary.
In a study cited in the afore-mentioned article, calcium blockage of the kidney tubes was discerned in wild-caught fish maintained in harder water; upon death the fish were dissected, and it was noted that the longer they managed to survive, the worse were the blockages. No mention is made of the actual GH, but the authors do note that a GH of 4 dGH was successful for maintaining the fish, though they would not spawn in this.
The late Dr. Jacques Gery once wrote that this species will easily live more than 10 years if maintained in very soft water. The fish never lives this long in medium hard water.