Sodium thiosulfate is used for the neutralization of chlorine and/or Iodine solutions at your hatchery or aquaculture facility.  Chlorine is highly toxic to fish and crustaceans so, whether disinfecting tanks or equipment with sodium hypochlorite (bleach) or using tap water during transport or stocking, it is crucial to remove chlorine prior to the introduction of your stock. 

For the neutralization of Chlorine solutions:

The amount of sodium thiosulfate required can vary with the pH of the water.  A range of approximately 2 to 7 parts sodium thiosulfate to neutralize one part chlorine is generally suggested. To neutralize 1 liter of a 200 ppm chlorine solution, approximately 0.4 - 1.4 grams of sodium thiosulfate would be required.  To neutralize one gallon of a 200 ppm chlorine solution, approximately 1.5 - 5.3 grams would be required

The use of a chlorine test kit is recommended.