Chloramine in aquarium water is a quieter problem than chlorine. It doesn't gas off if you let a bucket sit, ordinary carbon barely touches it, and when you do neutralize it, it leaves ammonia behind. More than one in five Americans drinks chloraminated water (EPA), and the City of Houston reports chloramine residuals in its 2025 water quality report. If you keep fish on municipal water, assume chloramine until your utility tells you otherwise. Here is what it is, what each removal method really does, and how to change water safely.
What chloramine is, and why your utility uses it
Chloramine forms when a utility adds ammonia to chlorinated water. The chlorine and ammonia combine into monochloramine, a disinfectant that lasts much longer in miles of pipe than free chlorine does and produces fewer disinfection byproducts (CDC). The EPA's maximum residual disinfectant level (MRDL) for chloramine is 4.0 mg/L, measured as chlorine. Compliance is judged on a running annual average, though, so individual samples can run higher (40 CFR 141.133). Houston's 2025 water quality report shows an average of 2.8 mg/L and a maximum of 4.8 mg/L in its main system (City of Houston). Treat 4 mg/L as high-normal, not a ceiling, and dose with a margin. Seachem, for example, says a double dose of Prime is safe for exceptionally high chloramine (Seachem).
What makes chloramine good for utilities is exactly what makes it bad for aquariums: it's stable. Unlike chlorine, which gasses off from standing water over a few days (CDC), it doesn't dissipate reliably. It can take weeks, so letting water sit isn't a reliable way to remove it. Boiling doesn't remove it either, and neither do reverse osmosis units or water softeners on their own (Nebraska Extension G1704; NJ American Water).
Why chloramine in aquarium water hurts fish
People drink chloramine without harm because it's neutralized in the digestive tract before it reaches the blood. Fish don't have that buffer. Chloramine passes straight across the gills into the bloodstream and reduces the blood's ability to carry oxygen, so a poisoned fish is effectively suffocating (Nebraska Extension). Nebraska Extension advises keeping chloramine below 0.1 ppm in water used for fish. Treat that as an upper limit, not a target. The EPA's national aquatic-life criteria for total residual chlorine in fresh water are much lower: 0.019 mg/L as a one-hour average and 0.011 mg/L as a four-day average (EPA). Those were set to protect sensitive wild species, but they show how little it takes. The EPA also notes that chlorine and chloramine can prevent the growth of the beneficial bacteria a tank depends on (EPA). The practical answer is a properly dosed conditioner every time, not a tap reading that seems low enough.
The signs look like any oxygen emergency: fish at the surface, fast gill movement, and fish lying still or darting after a water change. When that happens within an hour or two of adding tap water, suspect the water before you suspect a disease.
What a water conditioner actually does to chloramine
A water conditioner (dechlorinator) doesn't remove chloramine. It breaks it apart. The active ingredients are reducing agents, with sodium thiosulfate being the classic one. They destroy the chlorine half of the molecule, and that releases the ammonia half into your water (Holmes-Farley, Reefkeeping; Nebraska Extension).
It's worth putting a number on that ammonia. Monochloramine (NH₂Cl) has one nitrogen atom for every chlorine atom. Utilities report chloramine as chlorine (Cl₂, 70.9 g/mol), and ammonia test results are often given as nitrogen (14 g/mol). So every 1 mg/L of chloramine carries about 0.2 mg/L of ammonia-nitrogen (14 ÷ 70.9 ≈ 0.20). Water at 3 mg/L therefore releases about 0.6 mg/L of ammonia-nitrogen once it's treated (about 0.7 mg/L if your kit reads as NH₃). Treat that as a minimum. Utilities typically feed chlorine and ammonia at a ratio of chlorine to ammonia-nitrogen, 3:1 to 5:1 by weight (EPA). Free ammonia is almost gone only at 5:1 (EPA), so at lower ratios some ammonia is already in the tap water before you treat it. Test treated tap water once to find your real number.
What happens to that ammonia next depends on your tank:
- In a cycled tank, the biofilter eats it. With a 30% to 50% water change, the added load is a few tenths of a ppm of ammonia-nitrogen spread through the tank. A mature filter handles that, the same way it handles what your fish produce. If you're not sure your filter is up to it, read Beneficial Bacteria in Aquariums.
- pH decides how toxic it is in the meantime. At pH 8.0 and 79°F, about 5.7% of total ammonia is in the toxic un-ionized form. At pH 7.0, it's about 0.6% (UF/IFAS FA-16). Hard, alkaline tap water is where the leftover ammonia matters most. See Ammonia in Freshwater Aquariums and pH, GH and KH.
- "Ammonia-binding" conditioners hold that ammonia in a less toxic form for a while. The effect is temporary. Seachem, for example, says Prime holds ammonia for up to 48 hours and can be redosed every 24 hours if levels are rising (Seachem knowledge base), and independent data on binding times is thin. Either way, bacteria still have to process the ammonia. Binding buys time. It doesn't remove anything.
- In a new tank or a quarantine tank with no biofilter, nothing removes it. Freshly treated chloramine water can push an uncycled tank's ammonia up by itself.
Read the label, too. A plain chlorine remover such as sodium thiosulfate does break chloramine apart, but it does nothing about the ammonia that's released, and its label dose may not be set for chloramine levels (Holmes-Farley). Nebraska Extension simply advises that chemicals meant only for chlorine won't do the job. A product that says "chloramine" and is dosed for your water is the minimum.
A caution about your test kit
Ammonia kits use one of two chemistries, and it matters which one you have. Nessler-reagent kits give false-high or off-scale readings when an ammonia binder is in the water (UF/IFAS FA-16; Seachem). Salicylate kits are the better choice. UF/IFAS says binders don't affect them, while Seachem advises reading a salicylate test right away, because the test itself slowly breaks the bound complex apart and pushes the reading up. Neither kind tells you how much of the total is the toxic free form. For that you need the pH-and-temperature figures above, or a free-ammonia sensor such as Seachem's gas-exchange test. Test before the water change to get your baseline, then again a day later, rather than trying to read the result minutes after dosing.
Carbon: standard versus catalytic
A common claim is that a carbon filter takes care of chloramine. It mostly doesn't. Standard activated carbon removes chlorine quickly, but chloramine only slowly. It needs a slow flow rate, long contact time and fresh media to do the job (Water Conditioning & Purification, 2009; Nebraska Extension). Carbon breaks chloramine down into chloride, nitrogen gas and ammonia. It doesn't hold on to the ammonia, so that part passes straight through (Holmes-Farley).
Catalytic (surface-modified) carbon is made for this job and breaks chloramine down far faster than standard carbon (WC&P, 2012). If you pre-filter tap water for a large system, use a cartridge rated for chloramine, run it at the flow rate on the label, and replace it by its rated gallons rather than by the calendar. A bag of carbon in your canister filter isn't a chloramine plan.
RO, softeners and other things that don't solve it
- Reverse osmosis: an RO membrane alone doesn't remove chloramine (NJ American Water; Nebraska Extension). If you run RO for soft-water fish such as discus, make sure the prefilter stage is rated for chloramine, and test the product water for total chlorine.
- Water softeners: they don't effectively remove chloramine (Nebraska Extension).
- Standing, boiling, aerating: unreliable for chloramine (Nebraska Extension; CDC; NJ American Water).
Test for total chlorine, not free chlorine
Chloramine is "combined" chlorine. A free-chlorine test reads it wrong. Use a kit labeled for total or combined chlorine (Nebraska Extension; NJ American Water). Test at the tap once so you know your number, and test after any filter to confirm that it's working. Your utility's annual water quality report will also say which disinfectant it uses.
When the utility switches: free-chlorine "burns"
Some chloramine systems switch to free chlorine for a few weeks each year, mainly to clear the biofilm that builds up inside pipes (CDC). Gulf Coast Water Authority, for example, ran its conversion from April 6 to 20, 2026, and advised fish owners to keep using products that handle both chlorine and chloramine (GCWA). The water may smell and taste different, and hydrant flushing can cloud it. Don't change your routine. Keep dosing as usual, check total chlorine during the burn so you know the level, and don't assume standing water is safe just because the disinfectant changed.
A water-change routine for chloramine in aquarium water
- Know your number. Test total chlorine at the tap, or look it up in your utility's report.
- Dose for the new water. Treat the full volume of water you're adding. If you refill straight from a hose, dose for the whole tank volume before you start refilling.
- Match temperature before the water goes in.
- Change sensible volumes on uncycled or lightly filtered tanks. Several smaller changes beat one huge one when there's little biofilter to catch the released ammonia.
- Feed lightly on big water-change days, so the biofilter has one ammonia load to process instead of two.
- Test ammonia the next day, not five minutes after dosing.
For the wider picture of what's in tap water besides disinfectant, read Tap Water and Freshwater Aquariums.
FAQ
Does letting tap water sit remove chloramine?
Not reliably. Chlorine can gas off in a few days, but chloramine can take weeks. Use a conditioner labeled for chloramine.
Does a water conditioner remove ammonia?
No. Some conditioners bind ammonia temporarily, which lowers its toxicity for a limited time (up to 48 hours, by one manufacturer's account). Your biofilter is what actually removes it.
Will my filter carbon handle chloramine?
Poorly. Standard carbon needs long contact time and still leaves the ammonia behind. Catalytic carbon rated for chloramine is the carbon option that works.
How do I know whether my water has chloramine?
Check your utility's water quality report, or ask the utility. A total-chlorine reading well above the free-chlorine reading also points to chloramine.
Does chloramine matter when new fish arrive?
Yes. A shipped fish goes into your tank water, so that water should be treated, temperature-matched and cycled before the box arrives. When it does, float the sealed bag 15 to 30 minutes to match temperatures, add an ammonia binder the moment it's open, and switch the fish to your water with a measured flow over about 2 to 3 hours. See How Long Can Fish Survive in a Bag? for the full steps and why opening the bag is the risky moment, and what to expect when you buy fish online for how live-fish shipping usually works.
