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Common Salt, Sodium Chloride, NaCI

Common Salt, Sodium Chloride, NaCI is probably the oldest fish-keeper's treatment and standby, because it works well against many protozoan parasites. It can also assist osmoregulation problems, assist in boosting the protective mucus layer, support fish suffering from stress and help clear congested gills. Salt also inhibits the uptake of Nitrites by the fish, thereby limiting the damage to the red blood cells. Salt is a very safe pond treatment, and will not adversely affect biological filtration in pond filters. It is generally used at fairly high rates in short-term baths or dips, but can be used as a long-term supportive treatment in ponds.

Mostly, the cause of a disease outbreak can be traced to one or a combination of three factors that influence the health of the Koi. These factors are the environment, stress and pathogens. As describe in the introductory paragraph, salt assist to limit the influence of all three factors. The way it influence these factors are based on:


Koi maintain an internal concentration of salt in their body fluids higher than that of their liquid environment. Most fish have an internal salt concentration of around 1%. Tap water typically has close to 0.1% salt concentration. Osmosis causes water to transfer from the lower salinity of the pond water into the tissues of the fish. This additional water build up must be eliminated by the kidneys. Fish are remarkably well equipped to keep the internal salt concentration at the correct level, but the kidneys need to work around the clock to keep it like that. Under stressful conditions the fish must still maintain the salt level in the body fluids and this places additional stress on the fish. Increasing the salt concentration of the water slows the osmosis down and therefore the subsequent energy needed to maintain the internal salt concentration is less. When a fish suffers from bacterial ulcers, the damage to the skin allows more water to enter the fish and therefore salt plays an even more important therapeutic role.

Now just the opposite will happen to most parasites when salt is added to the water. So instead of water being drawn into the parasite, it will now be drawn out, dehydrating the organism. So to summarize, adding salt to water will assist the fish and stress out the parasites! Basically the parasite's cell dehydrates, and it will perish.

The following Parasites are likely to be killed by salt:

Trichodina (Although some salt resistant strains have been reported), Flukes (about 30%), Costia (30%), White Spot (Free swimming stage), Chilodonella, Epistylis.


Salt concentrations of 0.3% are effective at detoxifying nitrite. The salt interferes with the nitrite ion exchange at the fish's gills. Salt therefore inhibits the uptake of Nitrates by the fish, thereby limiting the damage to the red blood cells. Use salt until the nitrites have cycled properly.

Mild Bactericide.

Although not substantial, salt acts as a mild bactericide to reduce the bacterial load that the fish must cope with.
Stimulating Mucus production
Salt is useful in stimulating the mucus producing cells in the fish epidermis. When attacked by parasites fish, do much better with a heavy slime coat that is produced when adding salt to the pond system.

Benefits of salt

It does not harm your fish
It does not get bound out of the system like other treatments
There is no health risk to the hobbyist
Salt is inexpensive
Salt have a very mild influence on the bio-filter
Salt has been used to control algae at a concentration of .25% and higher.

Disadvantages of salt

Once salt is added to the pond, substantial water changes may be necessary to remove the salt from the pond
Salt may harm certain plants.
Killing large quantities of algae with salt may pollute the pond.
If fish are kept continuously in salt water, certain pathogens may develop a resistance to the effects of salt.
Keeping a koi permanently in salt solution can affect the chromatophores and dull colouration.

What salt to use

Use normal coarse salt without any additive like iodine or an anti-caking agent. Some anti-caking agents are toxic to fish. Although it is reported that iodine in salt may also be toxic to fish, my experience is that the negative effects of iodine on fish and filter bacteria is an old wifes tale. The iodene content of table salt is just too low to have any effect. During the years I have encountered two things with salt. One is that salt on its own does not lower the water’s capacity to carry oxygen to the extent that it will harm your Koi. Actually the dissolved oxygen content of water hardly registers a change after a mild dosage of salt was added to the system. In the middle of summer here in South Africa, I will recommend some extra aeration. The second point I would like to make is that I have never encountered fatalities when salt containing iodine has been used, not have I witnessed any any negative effect on the filter bacteria.


Pond treatment consists of 0.2% to 0.5% salinity. A 0.3% concentration is a good all around tonic for strengthening Koi and also as a preventative treatment, especially during spring and autumn. This concentration will stunt your water plants. You may choose to remove plants to a separate container during the course of the treatment. To dose at 0.3% salinity, three kg of course salt is added for every 1000 litres. It is preferable to add the salt at one kg per 1000 liters per day for three consecutive days, so the Koi and filter can adjust gradually. If it is unavoidable, the full dosage can be added. In fact the osmotic shock may just prove more effective against parasites. The only negative aspect is that the Koi will lose their appetite for a day. When adding salt to a pond, dissolve the salt in a bucket of pond water and distribute it evenly around the edges of the pond. Direct contact of salt crystals with the fish for more than a few seconds can cause injuries similar to burns.

A salt bath is more radical and sometimes nerve-racking but can be employed safely. This is done in a well aerated separate container. The dosage consists of 10 to 30 grams per litre. The best option is 20 gram per litre for up to 20 minutes. Remove the fish immediately if it thrashes around or lose equilibrium. A salt bath can quickly rid a fish of many parasites, bacteria or fungus. However, when the fish goes back into the pond, it may still be in a weakened condition and might fall prey to a new parasite.

Word of warning

Always adjust salt levels gradually over 3 days to allow the Koi time to adjust.
It has now also been acknowledged by many authors that the effectiveness of salt is getting less and less worldwide. This is mainly due to the frequent use of salt by breeders, dealers and hobbyists and the inevitable resistant pathogens to the effects of salt. One such example is the resistant Costia that we frequently encounter.

The immediate question that springs to mind is how can hobbyists create more resistant pathogens if they only add salt as a preventative treatment in spring or autumn?

The answer is not difficult to find if one reach for a calculator. Say for instance a 20 000 litre pond is treated with 0.5 percent salt. That means that initial dosages of 100 kg salt were added to the water. The dedicated pond keeper will do a 10 percent water change every single week. Remember that salt remain in the pond and can only be removed through water changes.

Week 1 remove 10%    Remove 10 kg     90 Kg in pond
Week 2 remove 10%    Remove 9 kg    81 kg in pond
Week 3 remove 10%    Remove 8.1 kg    72.9 kg in pond
Week 4 remove 10%    Remove 7.29 kg    65.61 kg in pond
Week 5 remove 10%    Remove 6.56 kg    59.05 kg in pond
Week 6 remove 10%    Remove 5.9 kg    53.15 kg in pond
Week 7 remove 10%    Remove 5.3 kg    47.85 kg in pond
Week 8 remove 10%    Remove 4.79 kg    43.06 kg in pond

In this table it is clear that after 8 weeks of 10% water changes, the pond keeper will still have a 0.21% salt content in the pond

Last Updated on Wednesday, 29 February 2012 12:48