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Chlorine and chloramine

By the koi.wiki editorial team · Updated

Key facts: Chlorine and chloramine are disinfectants added to municipal tap water that are highly toxic to koi and must be neutralized before the water enters a pond.[1][2]
Most public water systems disinfect drinking water with chlorine, and many add ammonia to form chloramine as a longer-lasting secondary disinfectant[3][8]. While these chemicals are effective at killing waterborne pathogens for human use, even trace amounts can be fatal to fish and other aquatic life[1]. Koi keepers must understand which disinfectant their water supply uses, because the two require different treatment methods before the water is safe for a pond[6][5].

What are chlorine and chloramine?

Chlorine is added to water supplies as a strong oxidizing disinfectant that kills bacteria, viruses, and protozoans by disintegrating cell walls and disabling intracellular proteins[2]. Chloramine, most commonly monochloramine (NH2Cl), is typically formed when ammonia is added to chlorine; it is used by water utilities since the 1930s because it provides longer-lasting disinfection as water travels through distribution pipes[3][8]. More than one in five Americans is served by chloraminated water[3]. Chloramine is less reactive and does not dissipate as quickly as free chlorine, which is part of why it is used as a secondary disinfectant[8].

Why these chemicals are dangerous to koi

Chlorine will kill fish and damage a pond's biological filter, since the same oxidizing action that destroys pathogens also harms the gill tissue and other organisms in aquarium or pond water[2]. According to University of Florida IFAS Extension, even the slightest trace of chlorine can prove fatal to fish and other aquatic life[1]. Chloramine, because it is formulated specifically to resist breaking down, lingers in water much longer than free chlorine and is also very toxic to fish and other aquatic organisms[1]. A Kodama Koi Farm resource notes that chlorine, chloramine, ammonia, and sudden pH swings can stress koi, damage their gills, and suppress immune systems[4].

Removing chlorine: aeration and sodium thiosulfate

Chlorine can be removed from water either by vigorous aeration, which allows the gas to dissipate, or by chemically neutralizing it with sodium thiosulfate (ST)[5][1]. University of Florida IFAS Extension describes a simple classroom activity in which adding sodium thiosulfate to a water sample brings chlorine down to 0.0 ppm almost immediately, while 24 hours of vigorous aeration greatly reduces chlorine but may not reach zero[1]. One suggested dosing ratio is roughly 7.4 ppm of sodium thiosulfate per 1 ppm of chlorine, though a separate aquaculture biosecurity publication cites a ratio near 7 mg of sodium thiosulfate per 1 mg of chlorine for disinfectant neutralization[5][7]. Texas A&M AgriLife Extension and the Southern Regional Aquaculture Center both note that if the chlorine source is liquid or gaseous chlorine, allowing water to sit in sunlight for about one week can dechlorinate it, a period that can be shortened to as little as 24 hours with air diffuser aeration[6][9].

Chloramine requires chemical dechlorination

Unlike free chlorine, chloramine does not dissipate readily through aeration or simple sodium thiosulfate treatment alone. University of Florida IFAS Extension states plainly that sodium thiosulfate and aeration do NOT remove chloramines, and that a commercial product must be used to break the compound down[5]. Texas A&M AgriLife Extension and the Southern Regional Aquaculture Center likewise advise that when the disinfectant source is chloramine, it must be removed by chemical dechlorination using commercial dechlorinators containing sodium thiosulfate or monosodium hydroxyl methane sulfonate[6][9]. According to Wikipedia's general overview, commercial dechlorinator chemicals act as reducing agents that convert chlorine species to harmless chloride, with compounds such as sodium thiosulfate, sodium hydroxymethanesulfonate, and sodium hydroxymethane sulfinic acid commonly used for this purpose[2].

Identifying which disinfectant is in your tap water

Because chlorine and chloramine require different treatment approaches, keepers are advised to contact their local water treatment facility to determine whether the disinfectant is gas, liquid chlorine, or chloramine before deciding on a dechlorination method[6]. A Kodama Koi Farm article on preparing for hot weather similarly advises pond owners to know whether their tap water contains chlorine or chloramine and to use an appropriate treatment method whenever topping up the pond[10]. Test strips can be used to check chlorine levels directly, and ammonia test strips can help indicate whether chloramine (chlorine plus ammonia) is present, since chloraminated water will continue to show measurable ammonia even after chlorine has been neutralized[1].

Practical dechlorination for ponds and water changes

Historically, aquarists could condition tap water simply by letting it stand for a day or two to allow chlorine to dissipate, but this approach became less reliable as utilities shifted toward monochloramine, which stays in the water much longer[11]. Commercial water conditioner products are formulated to neutralize chlorine, chloramine, and heavy metals found in tap water, and according to the US dealer Kodama Koi Farm, such products are generally recommended whenever new tap water is added to a pond, during routine water changes, or after heavy rainfall[4]. These stabilizers are described as a first line of defense against water chemistry swings that can stress koi, damage gills, and suppress immune function[4]. For broader context on routine pond water management, see water changes and the water-quality reference ranges page.Chlorination can also produce disinfection byproducts such as trihalomethanes and haloacetic acids when chlorine reacts with organic matter in the source water, and chronic exposure to some of these compounds has been linked to liver and kidney effects in humans, according to Wikipedia's overview of water chlorination[2]. Chloramine use can also increase lead leaching from older pipes because it is a weaker oxidant than chlorine and can destabilize protective lead pipe coatings, though water treatment plants can add corrosion-control chemicals to counter this[8]. These byproduct and corrosion issues are regulated for human health reasons, but they do not change the basic fact for koi keepers: both chlorine and chloramine must be neutralized before tap water reaches koi, which interacts with other parameters such as pH, KH (alkalinity), and the ammonia load already present in pond water during the nitrogen cycle.

Common questions

Can chlorine just be left to evaporate out of tap water before adding it to a koi pond?
If the disinfectant is liquid or gaseous chlorine, allowing water to sit for about a week (or as little as 24 hours with aeration) can dechlorinate it, according to Texas A&M AgriLife Extension and the Southern Regional Aquaculture Center[6][9]. This method does not work for chloramine, which requires chemical dechlorination[5].
Does sodium thiosulfate remove chloramine?
No. University of Florida IFAS Extension states that sodium thiosulfate and aeration do not remove chloramine, and a commercial dechlorinator product is required to break down the compound[5].
How much sodium thiosulfate is needed to neutralize chlorine?
One cited ratio is about 7.4 ppm of sodium thiosulfate per 1 ppm of chlorine, while a separate aquaculture biosecurity publication cites roughly 7 mg of sodium thiosulfate per 1 mg of chlorine[5][7].
How do I know if my tap water uses chlorine or chloramine?
Texas A&M AgriLife Extension advises contacting the local water treatment facility directly, since the appropriate dechlorination method depends on which disinfectant is used[6]. Ammonia test strips can also help, since chloraminated water shows measurable ammonia even after chlorine is neutralized[1].
Is chloraminated tap water unsafe for people but not just for fish?
The US EPA states that water containing chloramines which meets regulatory standards is safe for drinking, cooking, and bathing for humans[3], but the same water remains toxic to fish such as koi unless properly dechlorinated[1].

References

  1. ^ UF/IFAS EDIS FA171: Chlorine—Friend or Foe?. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA171. Retrieved November 2026.
  2. ^ Water chlorination (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Water_chlorination. Retrieved November 2026.
  3. ^ Chloramines in Drinking Water | US EPA. US Environmental Protection Agency. epa.gov/dwreginfo/chloramines-drinking-water. Retrieved November 2026.
  4. ^ How to Pick the Right Pond Water Conditioner. Kodama Koi Farm (koi dealer). kodamakoifarm.com/how-to-pick-pond-water-conditioner/. Retrieved November 2026.
  5. ^ UF/IFAS EDIS FA119: On-Farm Transport of Ornamental Fish. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA119. Retrieved November 2026.
  6. ^ Ornamental Fish Ponds and Water Gardens in Texas (Texas A&M AgriLife Extension). Texas A&M AgriLife Extension. extension.rwfm.tamu.edu/wp-content/uploads/sites/8/2013/09/Ornament…. Retrieved November 2026.
  7. ^ SRAC Publication No. 4707: Biosecurity in Aquaculture, Part 1: An Overview. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4707%20Biosecurity%20in%20Aquac…. Retrieved November 2026.
  8. ^ Monochloramine (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Chloramine. Retrieved November 2026.
  9. ^ SRAC Publication No. 435: Water Gardens. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/435%20Water%20Gardens.pdf. Retrieved November 2026.
  10. ^ How to Keep Your Koi Pond Cool During a Heat Wave. Kodama Koi Farm (koi dealer). kodamakoifarm.com/koi-pond-during-summer-heat-wave/. Retrieved November 2026.
  11. ^ Fishkeeping (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Fishkeeping. Retrieved November 2026.