Dechlorinator: How It Neutralises Tap Water
Why chlorine and chloramine in tap water harm fish and biofilters, and how a dechlorinating agent such as sodium thiosulfate neutralises them.
Overview
A dechlorinator is a chemical additive used to remove chlorine or chloramine from tap water before that water is added to an aquarium. The active ingredient is often sodium thiosulfate. It is a simpler, more targeted product than a full water conditioner when slime-coat protection or heavy-metal detoxification is not required.
Why chlorine is a problem
Chlorine is added to municipal water supplies as a disinfectant to kill bacteria, viruses and other microbes. The same oxidising action that destroys micro-organisms harms aquatic life: chlorine can kill fish and damage an aquarium's biological filter, where the nitrifying bacteria responsible for processing waste live.
Chlorine versus chloramine
Many water systems use chloramine, a more stable combination of chlorine and ammonia, instead of plain chlorine. A dechlorinating agent neutralises the chlorine portion; chloramine therefore requires a product rated for it, because removing only the chlorine can leave ammonia behind.
How it works
Dechlorinators act as reducing agents. Sodium thiosulfate reduces chlorine through an oxidation-reduction reaction: reacting with hypochlorite, the thiosulfate is oxidised to sulfate while the chlorine is converted to harmless chloride. The reaction is effectively instant once the agent contacts the water.
Where it is used
- Treating tap water before each water change
- Conditioning water for aquariums, spas and pools
- Treating settled backwash water at water-treatment plants before release
Practical notes
- Dose treated water before or as it enters the tank
- Confirm whether the local supply uses chlorine or chloramine
- A pure dechlorinator does not add slime-coat or detoxifying agents
- The reaction is rapid, so long pre-mixing is not required