Using Salt in Koi Ponds

There are many myths and half truths surrounding the use of salt in koi ponds. This essay deals with different viewpoints and realities.

Quick Info

Although salt has been used in aquaculture for many years the use of salt in a koi pond must be carefully considered by koi keepers.

Salt is not a cure all solution to all problems.

In the long term a low or high concentration of salt will not protect koi from any constant stressor.

If the pond environment has excellent water quality there is no need for the addition of salt.

Different levels of salt used in ponds must be carefully considered for DIFFERENT applications.

Salt is an irritant and will increase mucus production therefore the colours on a koi could be affected.

Constant exposure to salt can also negatively affect the colours on koi by acting on the pigment cells found in the dermis.

When koi are stressed they lose body salts because the functioning of the kidneys is impaired.

Salt can relieve stress - at specific levels. Salt can kill parasites - at specific levels and used in a specific way for a specific time. However, there are far better ways to target parasites.

The stronger the concentration of the salt, the shorter the time the fish should be exposed to it.

Nitrite toxicity in koi is lessened due to the chloride anion competing with the nitrite anion for entry into the gills.

Mucus production is stimulated on the skin AND on the gills when salt is added. This will have a negative effect on the intake of oxygen through the gill membrane.

Further, if there is a parasite infection present at the same time as irritation from adding salt the one irritation may make the other much worse with a consequent restriction on oxygen that can be absorbed through the mucus saturated gills.

The internal level of salt inside a koi is about 0.9%. Through osmosis a koi will gain water and lose ions (salts). Koi have evolved a natural mechanism to overcome this with a complex, constant exchange of salts, body fluids and intake of minerals via the gills, membrane in the gut and skin.

Part of this process is the kidneys. A koi's kidneys are very efficient at re-absorbing salts and quickly excreting excess water. Therefore, koi have adapted to live comfortably in equilibrium in fresh water. These processes are as natural as breathing in water to our koi.

Under certain circumstances the addition of salt can be beneficial to the koi.

Facts You Must Know

As a natural part of their existence freshwater fish, such as koi, are constantly dealing with an influx of water into the body and a loss of vital salts. Freshwater fish have evolved mechanisms to counteract this so there is a continual natural balance: they have extremely efficient kidneys which excrete water very rapidly and to offset the salt loss from the gills the efficient kidneys reabsorb salt from the urine.

The gills take up sodium ions in exchange for hydrogen ions and chloride ions for bicarbonate ions, thereby taking up salt from the environment and removing unwanted ions from the blood.

The collective name for these processes of maintaining the body salts concentration is called osmoregulation.

In a stressful situation or a parasite, bacterial, fungal or viral infection the essential processes of diffusion and osmosis starts to malfunction.

If a koi is placed in weak salt solution, there is a physiological action as the fish ceases to suffer water influx (water into the body) and ion efflux (ions lost out of the body). There is also a metabolic stimulation, so the action of this weak salt solution is generally one of health enhancement. The protection given by this level of salt is excellent for sick fish and is sometimes used in fish transport.

When higher levels of salt are used - 0.3% or 0.4% (3 kilos - 4 kilos per 1000 Litres) then the water influx and salt efflux are also reduced, relieving the sick, stressed or over-wintering fish in very cold areas.

This level of salt also enhances mucus secretion on the skin and on the gills. The mucus then provides some protection from parasites, bacteria, etc, and helps protect against colder temperatures. However this salt level is unnatural for the koi, as they have evolved to deal with life in freshwater. Water changes should be effected to reduce the salt levels to fresh water once treatment is finished.

If your biological filters have malfunctioned for some reason, and you are recording excessive levels of ammonia and nitrite, then the addition of salt to the pond can be extremely beneficial. Irritation due to ammonia can be relieved by stimulating the mucus i.e. increasing the protective slime coating.

Sea water has about 3.2 - 3.8% salt. 32 - 38kg per 1000L : 0.1% salt = 1 kilo of salt per 1000L. The maximum salt concentration koi should be subjected to is 0.4 - 0.5% (4 - 5kg per 1000 Liters). This should not be indefinite and water changes should be effected to reduce the salt concentration once the treatment is finished.

Salt can only be removed from the water by water changes not evaporation or topping up.

Long-term exposure to salt results in long term irritation and consequently long term stress. Higher concentrations of salt should only be used in short term dips or baths.

In order for salt to be effective against parasites the levels should be raised quickly and after a specific time water changes effected to reduce the salt solution or the koi returned to the pond (i.e. fresh water).

Salt concentrations can be raised quickly by transferring koi from fresh water directly to a basin/bowl or tank of prepared water that is salted - for a strictly limited time.

Fresh water fish and freshwater parasites subjected to sudden high levels of salt lose fluids rapidly and literally implode. Koi subjected to high levels of salt also lose fluids rapidly and can also die.

The first sign of going too far with salt baths or dips is koi losing balance and turning over in the water. Remove koi IMMEDIATELY to the pond or fresh water.

In new systems salt can help overcome the effects of nitrite poisoning as filters are starting up.

When koi have ulcers, missing scales or open wounds, using salt can be beneficial as there is an uncontrolled entry point for water into the body. The addition of salt will relieve the osmosis and prevent the loss of body fluids through the wounds. Low levels of salt give a balance between the body salts of the fish and the salt concentration in the environment. There will be far less ion fluxes at the wound site. This promotes healing and stops water accumulation at the wound.

As the wound heals the salt concentration should be gradually diluted down to zero once the fish is fully recovered.

The body fluids of the fish have a weak salt concentration, but the water in a freshwater koi pond has a significantly lower salt concentration. Water will tend to move into the koi across any semi-permeable surfaces such as the gills and the gut. However, not only is the water moving into the fish, but the salt ions gradually move out across the delicate gill membranes leading not only to water uptake but a salt loss also.

A strong salt bath or dip (or a strong salt solution added to the pond) acts in a slightly different way to that of the weak salt bath. The strong level of salt causes an osmotic pressure on the fish as salt ions try to diffuse into the fish and water is osmotically drawn out. The koi begin to dehydrate adding to any stressors already present. This pressure acts as a skin irritant, causes inflammation and leads to excessive production of mucus. This muco-hyper-stimulation causes some parasites, fungi, bacteria, and any other skin debris to be sloughed off.

Thus if the parasites are placed in a strong salt medium, then they will suffer diffusional ion uptake and osmotic water loss - they will dehydrate rapidly.

The parasites have a much lower isotonic point than the fish. Many single celled parasites are killed at 0.3% salting although some are beginning to show resistance (e.g. certain Asian Trichodinids). Larger parasites such as the flukes may need a higher level or longer duration. Visible macroparasites such as leeches or Argulus need strong hypertonic dips right at the limit of the fish's salt tolerance.

Generally, the stronger the salt bath the quicker the parasites will die.

Thus parasites are killed by water loss causing their cells to collapse. However, in a strong salt dip or strong salt solution in the pond it is not only the osmotic shock that kills the parasite there is also a direct toxicity from the sodium (Na) cation.

When sodium is taken up there is usually an equal amount of potassium or calcium cations taken up with it. However, in a strong salt bath, it is only the Na cation (and the Cl anion) that is greatly elevated. Thus Na accumulates in unusually large amounts, leading to toxicity in the parasite cells. This is why sea salts are less effective as high strength anti-parasite dips, as there is also elevated calcium and potassium present.

Koi have adapted to live in freshwater and have evolved mechanisms to deal with the problems this causes. If the fish becomes sick then elevating the percentage of salt to a mild level brings some physiological protection on the fish, and should reduce the parasite load.

Very sick or heavily parasitised fish can be placed in a short-term strong salt dip to rapidly kill parasites and promote mucous production.

It must be noted that all these effects on parasites also have an effect on our koi. In strong salt dips the timing is critical and the difference between a koi surviving or dying or even sustaining body damage is very fine.