Sodium Thiosulphate is the chemical that is used in bottles of dechlorinator that can be purchased at your local pet store, aquarium shop, garden centers, or pond store.   If you buy this pure form, it is dramatically cheaper.  All you have to do is dilute 500 mg (roughly 1 lb) with 1 gallon of distilled water.  This will treat around 40,000 gallons of pond water.  

     Removing chlorine and chloramine from tap water is very essential to your fishes health, as these compounds are very toxic.  This product can eliminate these toxins easily and quickly.