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.

the statement that "salt helps fish regulate their osmotic balance" is
patently ridiculous! all fish are different in this regard, depending on
where they live. some species are native to "soft water/low dissolved
solids" habitats while others inhabit hard and alkaline biotopes with high
salt concentrations inherent in the water. the species inhabiting these
very different environments have developed metabolic processes which take
advantage of the specific chemistry of that water. to add salt in the
misguided attempt to "help regulate osmotic balance" may actually upset that
very balance you are erroneously trying to "help". your fish are much
better served by leaving them alone and allowing them to regulate their own
osmotic balance ... something they have been doing without your help for
thousands of years.

The idea that salt is a tonic for fish when it is kept in fish tanks at
moderate levels (~ 1 tablespoon per 5 gallons) needs to be taken with a
"grain of salt". If your fish are the most commonly kept community fish
such as tetras, corys, angels or even the rasboras and most anabantids,
these are fish largely from soft, acid, low-TDS (total dissolved solids)
waters. The average tap water in the U.S and Europe is at least moderately
hard and alkaline and is certainly not improved by increasing the "salt"
concentration ... that very thing in which our water already differs most
from the natural waters of these fish. Certainly many of these fish adapt
well to our local water conditions and I am an advocate of adapting
non-breeding fish to local conditions. This is far better for both fish and
keeper than constantly battling see-sawing water parameters. But,
acclimation to your tap water is one thing .... making your water worse than
it already is however, is quite another.

The prophylactic use of salt to avoid common pathogens and parasites of fish
by keeping some level of salt in the tank is also fraught with difficulty.
True, the use of salt, usually accompanied by increased temperature, is an
effective treatment for one of the most common ectoparasite which beginning
aquarists encounter ... Ichthyophthirius multifiliis, or Ich. However,
only the free-swimming form of this parasite is treatable. Neither the
fish-embedded growth phase, nor the encysted multiplication stage that rests
on or in the substrate is readily attacked by salt or any other medication.
Thus prophylactic treatment is useless as it makes little sense to treat a
non-infected tank on a chronic basis.

Further, any treatment for parasitic infestation is a "medication" and thus
the use of salt differs little from the use of any one of the other
available Ick medications available at any LFS. The biggest difference in
my opinion though, is that of all the medications available, salt is
probably the least effective by comparison.

Finally, stimulating the slime coat through the use of an "irritant" makes
no sense to me. fish make a slime coat naturally and need no help from an
irritant which while potentially stimulating mucous production, also creates
significant stress which actually weakens the immune response.


--
You're never too old to have a happy childhood.