Using Sodium Thiosulfate to Dechlorinate Aquariums

A fast and economical way to dechlorinate water for your aquarium with Sodium Thiosulfate.


Maintaining a healthy aquarium environment requires careful management of water quality, and one of the key aspects is ensuring that the water is free from chlorine and chloramine, which are commonly found in tap water. Chlorine and chloramine are harmful to fish and other aquatic life, and dechlorination is essential before adding tap water to an aquarium. Sodium thiosulfate is a widely used chemical for this purpose. This article explores the properties of sodium thiosulfate, its mechanism of action, and how to use it effectively for dechlorinating aquarium water.

Properties of Sodium Thiosulfate

Sodium thiosulfate (Na2S2O3) is an inorganic compound with several important properties:

  • Chemical Structure: It is a colorless crystalline substance that is highly soluble in water.
  • Stability: It is stable under normal conditions and has a long shelf life.
  • Safety: It is relatively non-toxic to humans and animals, making it safe for use in aquariums.

Mechanism of Action

Sodium thiosulfate works by chemically neutralizing chlorine and chloramine in water. The reactions are as follows:

  1. Neutralizing Chlorine:Na2S2O3+Cl2→Na2S4O6+2NaC1 In this reaction, sodium thiosulfate reacts with chlorine to form sodium tetrathionate and sodium chloride (common salt).
  2. Neutralizing Chloramine:Na2S2O3+NH2Cl+H2O→Na2S4O6+NH3+HC1 For chloramine, the reaction is more complex. Sodium thiosulfate breaks down chloramine into ammonia and hydrochloric acid, which can then be managed in the aquarium.

How to Use Sodium Thiosulfate in Aquariums

  1. Calculate the Required Dose:
    • Determine the concentration of chlorine or chloramine in your tap water. This information can be obtained from your local water supplier or by using a test kit.
    • A general guideline is to use 0.1 grams of sodium thiosulfate to neutralize 1 ppm (part per million) of chlorine in 10 gallons of water. For chloramine, slightly more may be needed due to the additional step of breaking down ammonia.
  2. Dissolve Sodium Thiosulfate:
    • Measure the appropriate amount of sodium thiosulfate based on the volume of water to be treated and the chlorine/chloramine concentration.
    • Dissolve the sodium thiosulfate in a small amount of aquarium water before adding it to the tank. This ensures even distribution and effective dechlorination.
  3. Add to Aquarium:
    • Pour the dissolved sodium thiosulfate solution slowly into the aquarium or the new water being added.
    • Ensure proper mixing by gently stirring the water or using the aquarium’s filtration system to circulate the treated water.
  4. Monitor Water Quality:
    • After adding sodium thiosulfate, use a water test kit to check for residual chlorine or chloramine to ensure complete dechlorination.
    • Regularly monitor ammonia levels, especially if chloramine is present, as the breakdown of chloramine releases ammonia.

Advantages of Using Sodium Thiosulfate

  • Efficiency: Sodium thiosulfate is highly effective at neutralizing chlorine and chloramine quickly.
  • Cost-Effectiveness: It is relatively inexpensive and readily available in various forms, including crystalline and liquid solutions.
  • Safety: It is safe for use in aquariums when used in the correct dosage, posing minimal risk to fish and other aquatic organisms.


  • Overdosing: Avoid using excessive amounts of sodium thiosulfate, as it can deplete oxygen levels in the water and cause stress to fish.
  • Ammonia Management: When neutralizing chloramine, be aware of the resultant ammonia. Regular water changes and biological filtration can help manage ammonia levels.
  • Storage: Store sodium thiosulfate in a cool, dry place to maintain its efficacy.


Sodium thiosulfate is a valuable tool for aquarium enthusiasts, providing an efficient and safe method for dechlorinating tap water. By understanding its properties, mechanism, and proper usage, aquarists can ensure a healthy and safe environment for their aquatic pets. Regular monitoring and careful management of water quality are essential to maintain the delicate balance in an aquarium and promote the well-being of its inhabitants.

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