What it is:
Water gets sent to your house by a municipal water company. They get this water from different sources, many of which aren’t 100% germ free. The water gets treated with chlorine so the germs die. But when the treated water gets to your house some of that chlorine resides in the water, and can kill or seriously harm your fish.
Dechlorinator binds up the chlorine, making the water safe to use.
A stronger chlorine compound, called CHLORAMINE, is used by some municipalities. Dechlor works on these compounds too, by binding the chlorine fraction. A TINY, minuscule amount of ammonia gets left behind, to be used by your filter bacteria. If you do not HAVE filter bacteria, the ammonia from this could accumulate after a major water change to a level that might be measurable. People make a big deal about it, because making a big deal out of things gets a lot of attention.
Sodium thiosulfate is a compound which is used to remove and detoxify chlorine or other heavy metals which may be used by water companies to make water safe for consumption by humans. What it’s good at:
Dechlorinator is good for removing harmful Chlorine from new water which you’re adding to your pond during a water change.
Chlorine is often used by water-treatment plants to kill harmful bacteria and spores which may be in water intended for cooking or human consumption. The amount of chlorine used by the water authority will vary by the number of spores or cysts found on routine testing. When performing a water change, you should drain down the pond. Estimate the volume of water to be replaced and apply the dechlorinator to the pond. Then begin to refill the pond. Dechlorinator is non toxic. Dechlorinator cannot be overdosed (almost).