Common cooking salt ( NOT TABLE SALT) is a very useful addition to a koi keepers 'medical' cabinet as it has
numerous beneficial effects on fish. Not least of these is it's ability to counteract parasitic activity and promote healing.
Some koi keepers have a low level of salt in their ponds permanently, whilst others may add a low level over the winter months to help the fish survive the colder temperatures.
Other uses for salt include high strength anti-parasitic dips, the reduction of ammonia and nitrite poisoning.
The Biolology Bit
Salt is effective as a healing and anti-parasitic agent due to the biological processes of diffusion and osmosis.
Diffusion the movement of a subtstance dissolved in water (a solute ie. salt) across a semi-permeable membrane (ie cell wall), from an area of high concentration to low concentration. The diffusion stops where there is no concentration gradient existing. This means that the concentration is the same everywhere in the solution.
Thus in the case of a salt solution the sodium and chloride ions move across
the membrane to even out the two concentrations.
Osmosis is the movement of a solvent in this case water, through a semi-permeable membrane to an area of high solute concentration from an area of low solute concentration.
To explain this in more detail, if you imagine a strong salt water solution and another much weaker solution separated by a semi-permeable membrane. This membrane allows the passage of certain ions but not others. Therefore to even out these two solutions the water molecules pass across the membrane from the weaker solution to the stronger one until the salt levels in each are equal.
Simultaneously the salt ions diffuse across the membrane from the highly concentrated solution to the weaker one.
Relevance to Fish
Fish's body fluids have a weak salt concentration but obviously a freshwater pond has a significantly lower concentration than that.
Therefore, water will move into the fish across any semi-permeable surface/membrane. In fish the main membranes are the gills and the gut.
However, not only is the water moving into the fish, the salt ions are moving out across the gills also, so there is water uptake and salt loss occurring at the same time.
We can see from this that a freshwater fish is constantly having to balance an influx of water and a loss of vital salts.
Fortunately the fish have evolved to cope with this otherwise they would soon die. They are able to excrete water very rapidly due to their highly efficient kidneys. Also to deal with salt loss from the gills, the kidneys are able to reabsorb salt from the urine.
The gills take up sodium ions in exchange for hydrogen ions, and take up chloride ions for bicarbonate ions. Meaning they take up salt from the environment and remove unwanted ions from the blood. If hydrogen and bicarbonate ions were allowed to accumulate in the body they would cause the blood pH to drop.
The name for this process of maintaining body salts at the correct concentration is osmoregulation.
Providing the fish is healthy these processes function normally. If the fish becomes stressed,
parasitised, etc, then these essential processes can start to malfunction.
How Salt Protects Koi
A salt solution that has the same concentration as a fish's body fluid is called isotonic. This would be 0.9% or 11/2 oz per gallon.
A solution more concentrated than the fish's body fluids is called hypertonic.
One that is less concentrated would be termed hypotonic.
A hypotonic salt bath, 1/2 oz per gallon (0.3%), is the level used by some koi keepers to overwinter their fish as the concentration level between fish and water is greatly reduced. Therefore, the water influx and salt efflux are also reduced which places less stress on the fish, especially when sick or overwintering.
Another advantage of this level of salt is that it enhances the mucus secretions, this mucus then helps to defend the fish against parasites, bacteria, and the colder temperatures.
However this salt level is unnatural for the koi, as they have evolved
to deal with life in freshwater.
An isotonic salt bath can be beneficial to a koi suffering from missing scales, ulcers or other wound to the skin. This provides a balance between the fish's body salts and the concentration in the bath so that there are very few ion fluxes at the site of the wound. This promotes healing and also stops diffusion and osmosis at the wound site. The concentration can be dropped to hypotonic (0.3%) as the wound heals. Once fully recovered the level should be reduced to 0%.
In a weak hypertonic salt solution there is a physiological action as water influx and ion efflux cease. There is also a metabolic stimulation so a hypertonic bath is generally a health enhancement. This level is excellent for sick fish and may be used for fish when being transported. A hypertonic bath should not last longer than 12 hours, with the level of salt at 1%. Generally the higher the concentration of salt the shorter the bath.
A dip in a strong hypertonic salt solution causes osmotic pressure on the fish as the salt ions diffuse into the fish and the water is drawn out by osmosis. The pressure acts as a skin irritant and causes some inflamation, however the production of excess mucus it also causes is the important factor. The hyper-stimulation of the mucus causes any parasites, bacteria, fungi and other skin debris to be sloughed off, this being very beneficial to the fish.
We can see that salt can be used beneficially with fish at a range of concentration levels, providing it is used correctly. If you are in any doubt, please consult your vet.
Killing Parasites
Salt is best known among koi keepers for it's excellent antiparasitic properties but how does this work?
It has been explained above how ions and water pass across semi-permeable membranes ie the fish's skin and gills, by diffusion and osmosis. Therefore, if the parasites are put into a hypertonic solution, they will alos be subjected to the same processes. However, the parasites have a much lower isotonic point than the fish. Many single celled parasites are killed at a concentration of 0.3%, although some such as certain Asian Trichodinids have been showing resistance.
The larger parasites such as flukes, may need a higher concentration or a longer exposure to the same solution.
Leeches or Argulus will need strong hypertonic dips, to the limit of the fish's tolerance to salt.
The stronger the salt bath/dip the sooner the parasites will die.
The parasites are killed by water loss which causes their celss to collapse. In a strong dip however, it is also the direct toxic effects of the sodium (Na) cation (a positively charged ion), as well as the osmotic shock that kills the parasite.
When sodium is taken into the celss there is usually and equal amount of potassium or calcium cations taken in with it. In a strong hypertonic dip however, it is the sodium cation that is greatly increased, therefore the sodium accumulates in large amounts and leads to a toxic build up in the parasites cells.
This is why natural sea salts are less effective in the treatment of parasites as they also contain higher levels of potassium and calcium.
Other uses for salt
Even the most vigilant koi keeper can have problems caused by the breakdown of a pump or filter. If a biological filter is not working efficiantly, or breaks down you will have higher levels of ammonia and nitrites in the pond. The addition of salt to the pond in these cases can be extremely beneficial. The irritation caused to the fish by an excess of ammonia can be relieved by stimulating mucus production, thus increasing the protective coating of slime on the fish. The toxic effects of nitrites can be lessened by the chloride anion (a negatively charged ion) competing with the nitrite anion to enter the gills.
Doseage
Warning. Must never be used with Formalin or Anaesthsetics.
Cannot be used with all chemicals, notably potassium and chloramine T at high strength. Max recommended dosage with these chemicals is ½ ounce per gallon (0.33%)
For use as a general supportive treatment and tonic ’ – ½ ounce per gallon (0.33%).
As a supportive when treating bacterial infection with other treatments, such as Acriflavine – ¾ ounce per gallon (0.50%).
As a dip – 4 ounces per gallon (2.64%) for 10 - 15 minutes OR until the koi shows signs of distress whichever is the shorter.
