liv2padl wrote:
actually, I don't think it has much scientific merit at all. In fact, it
goes against many of the principals of aquatic biology as we know them.

You forget one scientifically proven use for salt in fish: preventing
nitrite poisoning during cycling, particularly "brown blood disease". To
refresh your memory, nitrite can oxidate hemagoblin and create
methemoglobin. Methemoglobin is unable to transport oxygen. Too much
methemoglobin and the fish dies of suffocation. How is this mitigated? By
the Cl ions in salt (either NaCl or KCl). The Cl ions compete with the
nitrite ions for uptake by the gills and thus help prevent the creation of
methemogoblin by mitigating the uptake of nitrite. Thus you save your fish
from a nasty death via aphyxiation if you provide a little salt during the
nitrite spike of the cycle. I'm willing to bet this is one of the reasons
the use of a "prophylactic" level of salt took off, because less
scientific fish keepers noticed that salt tended to mitigate new tank
deaths without realizing why.