Dechlorinating Water for Koi Ponds: Methods and Products

By KoiQuanta Editorial Team|Updated Mar 2026

Chloramine requires a sodium thiosulfate plus an ammonia binder -- standard dechlorinators alone are insufficient. This is the dechlorination mistake that many koi keepers don't realize they're making. If your municipal water contains chloramine (increasingly common as water utilities move away from simple chlorination), adding sodium thiosulfate (the active ingredient in most standard dechlorinators) neutralizes the chlorine portion but leaves ammonia attached to it -- which then enters your pond as free ammonia and affects your fish.

Before treating your water, find out whether your utility uses chlorine or chloramine. This changes what product you need.

TL;DR

Chloramine: The Critical Difference Chlorine (Cl2): Simple chlorine gas dissolved in water.
Volatile -- it off-gases from water over 24-48 hours when water is left to stand in an open container.
Dose: 10-20 mg of sodium thiosulfate per liter of water per 1 mg/L of chlorine.
Most tap water contains 0.2-2 mg/L chlorine, so you need 2-40 mg/L sodium thiosulfate.
For a 500-gallon water change: 50 mL of Prime.
If you suspect chlorine/chloramine exposure: 1.
Immediately add dechlorinator at double normal dose 2.

Chlorine vs. Chloramine: The Critical Difference

Chlorine (Cl2): Simple chlorine gas dissolved in water. Volatile -- it off-gases from water over 24-48 hours when water is left to stand in an open container. Neutralized by sodium thiosulfate instantly.

Chloramine (NH2Cl): Chlorine chemically bonded to ammonia. More stable than chlorine -- it doesn't off-gas over time and is not fully neutralized by simple sodium thiosulfate. When sodium thiosulfate breaks the chloramine bond, chlorine is neutralized but ammonia is released.

How to find out which your utility uses:

Check your annual koi pond water quality tracker report (utilities are required to send these)
Call your water utility directly and ask
Many utility websites list their disinfection method in water quality information

KoiQuanta's water change log includes a dechlorination product field. Logging which product you use alongside your water change records creates a record of your water treatment practices, useful for troubleshooting any water quality issues that arise after water changes.

Products for Chlorine Treatment

Sodium thiosulfate (pure): Neutralizes chlorine instantly. Very inexpensive. Often sold as a bulk powder for ponds. Does NOT neutralize chloramine. Only use for chlorine-only water.

Dose: 10-20 mg of sodium thiosulfate per liter of water per 1 mg/L of chlorine. Most tap water contains 0.2-2 mg/L chlorine, so you need 2-40 mg/L sodium thiosulfate. Commercial pond dechlorinators typically provide this at their recommended dosing rates.

Ascorbic acid (Vitamin C): Also neutralizes chlorine rapidly. Safe, inexpensive, and doesn't add sodium or any concern about overdosing. Does NOT neutralize chloramine reliably.

Products for Chloramine Treatment

Sodium thiosulfate + ammonia binding combination products: Products like Seachem Prime, API Stress Coat, and similar formulations combine sodium thiosulfate to neutralize the chlorine component of chloramine with an ammonia binder (usually sodium hydroxymethanesulfonate or similar) that detoxifies the released ammonia. These are the appropriate choice for chloramine water.

Seachem Prime specifically: Temporarily detoxifies ammonia for 24-48 hours, allowing the biological filter to process it safely. This is the mechanism that makes these products useful for chloramine water -- they don't remove the ammonia, they neutralize it temporarily.

How much dechlorinator do I use per gallon?

For Seachem Prime and similar products: typically 1 mL per 40 liters (about 10 gallons). For large pond water changes, this adds up -- buy in larger containers. For a 500-gallon water change: 50 mL of Prime.

For sodium thiosulfate powder (chlorine only): follow your product's dosing for your estimated chlorine level. Most products are designed so one measured dose per volume is adequate.

Chlorine/Chloramine Exposure Signs in Koi

If you add chlorinated or chloraminated water without adequate dechlorination, you'll see:

Immediate flashing, erratic swimming, and surface gasping
Rapid mucus overproduction (protective response to chemical irritation)
Gill damage (chlorine is a powerful oxidizer that damages gill tissue)
In severe exposure: mortality within hours

This is a true emergency. If you suspect chlorine/chloramine exposure:

Immediately add dechlorinator at double normal dose
Do a 25-30% water change with properly treated water
Maximize aeration
Add activated carbon to the filter if available (adsorbs some residual chlorine)

Practical Application for Water Changes

Water change using a garden hose: The most practical approach for most pond keepers. Add dechlorinator to the pond before you start adding the hose water, or add it to a bucket and pour it in during the water addition. You don't need to pre-mix it -- dechlorinator acts instantly on contact with water.

For large water changes or new pond filling: Pre-treating water in a holding tank or barrel allows you to add dechlorinator, verify the treatment, and let any residual off-gas before adding to the pond. This is ideal for adding large volumes but more logistically complex than hose addition.

Temperature matching: Dechlorination doesn't affect temperature. Match temperature separately (see the koi water change guide) -- this is equally important for fish welfare.

For the chemical side of water treatment and the full water chemistry context, the koi water chemistry guide covers how dechlorination fits into overall water quality management.

Dechlorinating for Quarantine Tanks

Quarantine tank water changes happen more frequently and the tank volume is smaller, but the dechlorination requirement is identical. In fact, it's more important -- the quarantine tank biofilter is potentially less than your main pond filter, and any chlorine exposure can set back the bacterial colony considerably.

Always dechlorinate quarantine tank water changes, even if it feels excessive for "just" a small tank. Chlorine damage to the biofilter in a quarantine tank causes ammonia spikes that harm the fish you're trying to protect.

Frequently Asked Questions

How do I treat tap water for my koi pond?

First, determine whether your tap water contains chlorine or chloramine -- check your utility's annual water quality report or contact them directly. For chlorine: sodium thiosulfate or any standard dechlorinator works. For chloramine: use a product that combines a chlorine neutralizer with an ammonia binder (Seachem Prime and similar products). Add the dechlorinator before or during the water addition to the pond. Dose accurately based on your product's instructions and your water change volume.

What is the difference between chlorine and chloramine for koi?

Chlorine is simple dissolved chlorine gas, volatile and easily neutralized by standard dechlorinators (sodium thiosulfate). Chloramine is chlorine chemically bonded to ammonia, more stable than chlorine, and not fully neutralized by sodium thiosulfate alone. When sodium thiosulfate breaks chloramine, the chlorine is neutralized but the ammonia is released -- requiring an ammonia binder component. Using a chlorine-only dechlorinator in chloramine water leaves ammonia in your pond water, creating a hidden water quality problem. Many utilities have switched to chloramine, so check before assuming your old dechlorination routine is still adequate.

How much dechlorinator do I use per gallon?

Follow your specific product's dosing instructions. For Seachem Prime: approximately 1 mL per 10 gallons (40 liters). For most commercial pond dechlorinators: check the label for the stated dose per gallon/liter and apply accordingly. When in doubt, slightly overdosing a dechlorinator like Prime is safe (the ammonia-detoxifying effect has some excess capacity). Never add chlorinated or chloraminated water to a koi pond without dechlorination first -- the gill damage from even brief chlorine exposure can be severe.

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Sources

Associated Koi Clubs of America (AKCA)
Koi Organisation International (KOI)
University of Florida IFAS Extension Aquaculture Program
Fish Vet Group
Water Quality Association