Use only rock or pool salt never iodised salt.
Pool or Rock salt is not just salt (sodium chloride). It is made from evaporated seawater.

It is used in the health care and maintenance of fishes especially Koi. Salt is useful in treating parasites, it will eliminate seven out of nine parasites that are commonly found in Koi ponds.
Most fish have an internal salt concentration of 1.0% or 10 parts per thousand, which is higher than their water environment. Osmosis causes water to transfer from the lower salinity of the pond water into the tissues of the fish. Addition of salt will lower the osmosis pressure. This reduces the effort of the fish in eliminating the excess water. The saved energy is then available for use by the fish's own immune system to fight diseases.

Pond fish actively maintain a natural balance of electrolytes in their body fluids. Electrolytes such as potassium, sodium, chloride, calcium and magnesium are removed from the water by chloride cells located in the gills. These electrolytes are essential for the uptake of oxygen and release of carbon dioxide and ammonium across the gill membranes. The lack of electrolytes can cause health problems in pond fish. Adding salt provides essential electrolytes and trace elements fish need to survive.
Salt helps improve gill function to reduce stress.
During periods of disease and stress, healthy gill function is disturbed. This can lead to the loss of electrolytes through the gills, called osmotic shock. Osmotic shock reduces the intake of oxygen and the release of carbon dioxide and ammonium from the gills. Salt reduces the risk of osmotic shock by replenishing natural electrolytes fish need.

Overcrowding and overfeeding can lead to high nitrite levels, especially in newly set-up ponds.
The nitrite ion N02- enters the gills and prevents the blood from carrying oxygen, resulting in nitrite toxicity. The addition of salt will reduce the toxic effect of nitrite.

Salt concentrations between 0.3% to 0.5% will upset the osmotic balance of some parasites.
A concentration of 0.3% is effective at detoxifying nitrite and 0.25% or higher has been used to control string algae.

Once added to a pond, salt does not evaporate and is not filtered out. Salt should be added each week (ie. when the settling tanks are cleaned and emptied), when topping up a pond, unless the test results indicate salt is not needed.

When adding salt to a pond for the first time, add small amounts over a 7-day period to allow the filter bacteria to adjust.
Many Koi enthusiasts advise adding salt around the parameter of the pond. The author advises placing the salt in a corner, so the fish can then select their own concentration.

The recommended dose is between 0.1% (100gram per 100lts) and 0.3% (300grams per 100lts).
Using a hydrometer a specific gravity at 0.3% will give a reading of 1.002.

Testament of salt.
The author has seen fingerlings and large Koi hovering over the pile of salt.
After the 2005-spawning season a large Kohoku injured itself on a venturi in the main pond, a 10cent-size wound appeared between the lateral line and dorsal fin, within 5 days the wound was healed completely without any medication being needed.

As a foot Note
The author has added 3kg of pool salt each and every week to a 50,000lt pond after cleaning the 4 settling tanks and refilling with 3,000lts of fresh water. Over a period of 14 years the specific gravity is still at 1.002.


Article by
Harry Watson