Hej,
Detta är den bästa texten jag hittat hittills. Den förklarar varför, när och hur det fungerar.
SALT
MECHANISM OF ACTION:
Fish cells are saltier on the inside than the fresh water outside. When there is unequal concentrations of salts, the area with higher salts will lose salt to the area with lower salts. At the same time, water will move from the area of lower salts into the area of higher salts to dilute the salts. This is called osmosis. It is the reason that blood on clothes is removed by soaking in plain water ... it lyses the blood cells and dilutes the iron out of the cloth.
All animals that live in fresh water have to expend energy to hang onto their salts AND to keep the fresh water out. Most of the fresh water fish can no longer deal with "salt water" conditions (altho ocean going salmon can go back and forth from salt to fresh water).
But adding some salt to the tank water lowers this energy expenditure. Salt also does one other thing, it stimulates the production of slime. Fish dont have much in the way of antibody, but they do have a secretory kind that is added to the slime coat where it reacts with parasites. But having a continuous production of slime, parasites have a harder time getting to the skin of the fish and shedding of the slime coat sheds many of the parasites as well. Normally, healthy fish are resistant to even pathogenic strains of bacteria. However, fish suffering mechanical damage from handling or spawning that removes the slime coat and/or opens a wound, those that have a primary infestation with a parasite that opens the slime coat and punctures the skin of the fish and when water quality/oxygen levels/temperature are so poor that the immune system of the fish is dysfunctional or there are infected fish shedding large numbers of bacteria in the pond can result in bacterial infection.
What salt dips do:
1. Helps fish that have been shipped recover their electrolytes
2. Strip off the slime coat and chemically knock off a lot of parasites.. it seems to be more of a shock to parasites than the fish
3. A fish that has been dipped is more susceptible to medications once the slime coat protectant is gone AND the remaining parasites are exposed to medications.
What low concentrations of salt does:
1. Provide needed minerals (if solar salt is used)
2. Stimulates slime coat production. Fish have antibodies and other anti microbial agents which are excreted into the slime coat where they bind to parasites.
3. Appears to protect fish against nitrite poisoning
CONCENTRATIONS:
Percent (%) is a measure of the number of grams of a chemical per 100 ml of water. So 0.3% salt solution means there is 0.3 grams of salt per 100 ml of water.
THIS HAS NOTHING TO DO WITH WHAT IS READ ON A SALT METER - please read the instruction manual that comes with the meter to translate what the METER says with what the concentration is in percent (%)
A solution of one teaspoon per gallon is 0.132% salt. So 1 tablespoon per 5 gallons is around 0.1%
A LOW concentration is up to 0.1%. This concentration will not hurt most plants and is what is typically used in ponds and tanks. Addition to fresh water is done over 3 days.
A MEDIUM concentration is up to 0.5% . Addition from 0.1% up to 0.5% should be done over a couple of days.
HIGH concentrations are up to 0.9% which is isotonic or the concentration within the fish. This concentration is used for medical reasons for very short baths. THIS CONCENTRATION CAN BE DEADLY TO FISH.
mild nitrite protection = 0.1% (3/4 teaspoon per US gallon)
preventive and nitrite protection = 0.3 % (2 1/2 teaspoons/gallon)
What kind of salt
Noga recommends "solar" salt WITHOUT ANTI-CAKING additives. The additive is sodium ferrocyanide (yellow prussiate of soda) which makes hydrogen cyanide when exposed to fish. On the topic of salt, p. 295 talks about salt dips for treatment of ecoparasites, columnaris and bacterial gill disease, and how salt dips remove the excess mucus making medications more effective against pathogens. Solar salt can be bought in food stores, in crystal form, no additives for water softening in big bags for cheap. Rock salt dissolves slowly. This means it will not flash burn the gills or skin like adding finely crushed table salt will. Almost all GF books specifically say no iodine.
http://www.mu.edu/~buxtoni/puregold/disease/treatment/trtmnt.htm#salt
To learn more about:
salt to prevent nitrite toxicity:
http://edis.ifas.ufl.edu/scripts/htmlgen.exe?DOCUMENT_VM007
http://www.coloradokoi.com/nitrific.htm
salinity and electrolytes: page 4 of:
http://ag.ansc.purdue.edu/aquanic/publicat/usda_rac/efs/srac/464fs.pdf
osmoregulation in fish:
http://www.britannica.com/bcom/eb/article/8/0,5716,120748+6+111049,00.html
salt for biofilter bacteria: see Table 3 on page 5 of:
http://agpublications.tamu.edu/pubs/efish/452fs.pdf
calculating how much salt to use:
http://ag.ansc.purdue.edu/aquanic/publicat/usda_rac/efs/srac/410fs.pdf 
