Clown Loaches

[mikev]

Salt is a pollutant to most freshwater fish, and particulaly seriously so for scaleless fish where it readily crosses the fishes skin membranes upsetting the osmotic balance and overworking the kidney, which burns out killing the fish. Freshwater fish living in tanks with salt always have a residual level of stress, and rarely live anything like as long as their counterparts in un-polluted tanks.

OK, you got me really interested here, so I did a bit of checking.

Here is the apparent picture: osmotic pressure is certainly a factor, but it seems to work in the opposite direction. Fish naturally contains salt in its own liquids (blood!) and since a fish is an "open system", osmotic pressure will try to equalize the concentrations. This is accomplished by passing water through the skin.
For a freshwater fish, osmotic pressure moves the outside water into the fish; fish compensates by using its kidneys to expel extra water. For a saltwater fish, the pressure is reversed and fish would compensate by drinking water.

As for passing NaCl through the skin membrane: it happens, but in the opposite direction from what you stated: The article linked below says

Salts diffuse from areas of high concentration (blood)
to areas of low concentration (fresh water).
Therefore, salts (primarily sodium and chloride)
are slowly but continuously lost (osmotic
leakage) to the environment.

Now, if we put small amount of salt into a freshwater tank, it will *decrease* the osmotic pressure and thus decrease the load on kidneys. However, if we put more salt we will eventually reverse the direction of the osmotic pressure (and this will probably kill the fish very quickly).

The question is just how much salt is in fish bloods. The key number can be found, for example in this article:
http/www.ca.uky.edu/wkrec/SALTTRANS.pdf
Quoting:

Fish and other vertebrates have a unique
and common characteristic. The salt content of
their blood is almost identical. Vertebrate blood
has a salinity of approximately 9 g/l (a 0.9% salt
solution) and a pH of 7.4. Approximately 77%
of the salt in blood is sodium and chloride.

In other words, 1% salt solution will be bad indeed.

The "recommended" dosage of freshwater salt is a teaspoon (5g)/5 gallons, see for example, here:
http/www.aquariumfish.net/information/aquarium_salt.htm

This is way below the reversal level. The 10% of the recommendation that I quoted (5g/50g) represents a very minor change in osmotic pressure and as far as kidneys go, seemingly in positive direction.


Now, it is fully possible that there are other arguments against salt (not skin irritation and not osmotic pressure): Live plants is one, surely, I have no idea what low concentration of salt would do to them. But since there are clear pro-salt arguments too (no ich worries, for one), it does not seem to be a clearcut "no salt" choice.

Hopefully this is of interest.