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Sodium Thiosulfate (Dechlorinator)


Chlorine is a toxic element, and excess chlorine could be harmful to fish and crustaceans, so it is important to remove chlorine from the water before introducing the stock.

It is stated that even as little as .05 parts per million (ppm) of chlorine in your pond may result in fish deaths. Others say, you can add 10% chlorinated water to your pond without any worries. But if your source water has a chlorine content of 0.5 ppm, a typical value, and you add 10% to your pond, the pond concentration of chlorine is .05 ppm - a toxic level!

I have tried to establish the parts per million of chlorine from our area’s water service provider. Unfortunately the dosage is not very consistent and the parts per million of chlorine in the water will depend on the distance of the water network from the purifying plant to your house, as well as the water consumption in the neighbourhood. The higher the consumption, the more chlorine will be present in the water. It is therefore obvious that every time you fill your pond or top it up directly from a hosepipe there is a considerable risk of harming or even killing your Koi.

If your water source contains other impurities such as heavy metals, it will be best to install an activated charcoal filter in the supply line. If your only concern is chlorine in the water source that you use, Sodium Thiosulfate (also spelled Sodium Thiosulphate) will neutralise chlorine instantly. Sodium Thiosulfate is an inexpensive way to neutralise chlorine in large water volumes like koi ponds. It is also harmless to koi, so it can be overdosed without negative effects.

Sodium Thiosulfate is also an excellent neutralising agent to use in the accidental overdosing of Potassium Permanganate.

Like mentioned before, Sodium Thiosulfate is harmless to koi and painstaking measuring of the dosage amount is not as critical as with potential toxic medication. As a rule of thumb I dissolve one teaspoon of crystals in a bucket of tap water, for every 1000 litre of pond water to be replaced.

Some water service providers in South Africa will use chloramine in the water as an alternative to prevent waterborne disease. If the water contains chloramine, sodium thiosulfate will break the chlorine-ammonia bond. Sodium Thiosulfate will then neutralize the chlorine, while the filter system will take care of the traces of ammonia that will remain in the water.

The same will however happen when a hobbyist use the old trusted chloramine T to treat parasites in a pond after water changes. The excess Sodium Thiosulphate will neutralize the chlorine and leave only ammonia in the water. In effect the treatment will be useless but the hobbyist will be under the impression that he/she gave the parasite a nasty blow. Use chloramines T only a few days before adding Sodium Thiosulphate or leave enough time after adding the Sodium Thiosulphate for it to disappear.

Duncan Griffiths has put it very eloquently: “yes S/T  totally screws chloramine T up”!


Sodium Thiosulfate is a great product and the most economical one I am aware of! Remember, it should be added to the water before any fresh water is introduced!

Last Updated on Wednesday, 29 February 2012 12:56