Chlorine & chloramines:
the danger hiding in your tap

Ask a new fishkeeper what they worry about and the answer is almost always the same: ammonia, then nitrite, then nitrate. These are the stages of the nitrogen cycle — the biological process that every tank goes through before it settles into a healthy equilibrium. They matter. But they take weeks to develop, they build gradually, and a basic test kit catches them before they become lethal.

Chlorine and chloramines don't work like that. They're present in your tap water right now, at concentrations carefully calculated to kill microorganisms. When that water goes into an aquarium without treatment, the same chemistry that makes it safe to drink makes it acutely toxic to fish. There is no warning period, no gradual build-up, no time to test and respond. The damage happens on contact.

Understanding what these compounds are, how they work, and how to deal with them correctly is one of the most important things an aquarist can know — and one of the most commonly misunderstood.

What water companies actually add — and why it matters which

Drinking water in the UK is disinfected. That disinfection is not optional — without it, waterborne diseases that killed people in the nineteenth century would return. The two main approaches are chlorination and chloramination, and which one your supplier uses has significant practical consequences for your tank.

Free chlorine

Free chlorine (Cl₂, or more precisely hypochlorous acid and hypochlorite ion at drinking water pH) has been used for over a century. It's effective, it's cheap, and it's well understood. In a tank, free chlorine is a powerful oxidant: it attacks biological membranes, destroys proteins, and reacts with organic compounds. It does this indiscriminately — fish gill tissue is no more resistant to it than the bacteria it was added to kill.

Free chlorine does have one property that aquarists used to rely on: it's volatile. If you fill a bucket with tap water and leave it in a well-ventilated space for 24 hours, most of the free chlorine will gas off. Vigorous aeration speeds this up considerably. This is why older fishkeeping guides recommended "ageing" tap water before use. It worked — for chlorine.

Chloramines — the modern replacement

Over the past two decades, a large number of UK water suppliers have switched from chlorine to chloramines — most commonly monochloramine (NH₂Cl), formed by combining chlorine with a small amount of ammonia. The reason is purely practical: chloramines are far more stable than free chlorine. They persist through the distribution network, maintaining residual disinfection all the way to your tap without the repeated dosing that chlorine requires. For water companies, this is an improvement. For aquarists, it is a serious complication.

The other complication is chemical. When chloramine is reduced — as happens in biological filter media, or during certain dechlorination reactions — it releases its ammonia component into the water. An untreated chloramine dose that a filter manages to neutralise can produce a measurable ammonia spike as a consequence. The fish face a two-stage threat: oxidative damage first, then ammonia poisoning from the same compound.

Why these compounds are more dangerous than ammonia or nitrite

The comparison to ammonia and nitrite is worth dwelling on, because aquarists are conditioned to think of those as the primary threats. They are real threats — but they operate on a timescale that gives you options. An ammonia spike from an uncycled tank builds over days. A nitrite crash during a cycle is predictable and visible on a test kit. You have time to do a water change, add a bacteria supplement, adjust your feeding.

Chlorine and chloramines are acutely toxic at concentrations far below anything that triggers visible symptoms until it's too late.

The mechanism is direct and fast. Chlorine is a strong oxidising agent. When water containing it contacts fish gill tissue — the thin, highly vascularised membranes through which fish breathe — the oxidative damage begins immediately. The gill epithelium swells, producing excess mucus as a defensive response. As damage progresses, the effective surface area for gas exchange collapses. Fish essentially suffocate, not because there's no oxygen in the water, but because their gills can no longer absorb it. Simultaneously, chlorine reacts with haemoglobin to form methaemoglobin, a form that cannot carry oxygen at all, compounding the suffocation.

The onset of visible symptoms — clamped fins, erratic swimming, surface gasping — typically appears only after significant gill damage has already occurred. By that point, the prognosis is poor even if the water is corrected immediately.

Chloramines cause the same gill damage by the same oxidative mechanism, but with two additional problems. First, they are more stable in biological tissue — they penetrate more readily and persist longer at the site of damage. Second, as noted above, they carry ammonia in chemical combination, releasing it as they are metabolised or broken down by biological processes. A fish stressed by chloramine damage is simultaneously being exposed to rising ammonia levels in its immediate environment.

The biological filter

Fish are not the only casualty. The nitrifying bacteria that make a cycled tank function — Nitrosomonas and Nitrospira — are living organisms with cell membranes just as vulnerable to oxidative damage as fish gills. A large water change with unchlorinated water can crash a mature filter's capacity within hours. The tank may then experience an ammonia spike days later, when the fish appear healthy and the immediate crisis seems to have passed. This delayed secondary crash is one of the more insidious consequences of improper dechlorination.

Dechlorination — what works and what doesn't

There are four practical approaches to the chlorine and chloramine problem, and they differ significantly in what they actually neutralise.

Sodium thiosulfate — for chlorine only

Sodium thiosulfate (Na₂S₂O₃) is the original dechlorinator — the active ingredient in many older or cheaper products, and still sold as a standalone powder. It reduces free chlorine instantly and completely. At one time, it was all you needed.

It does not neutralise chloramines. When sodium thiosulfate reacts with chloramine, it breaks the Cl–N bond and releases the ammonia component directly into the water. Using sodium thiosulfate in a chloraminated supply doesn't make the water safe — it removes one toxin and replaces it with another. If your water company uses chloramines (and many UK suppliers now do), sodium thiosulfate is the wrong product.

Modern dechlorinators — for both

Products formulated to handle chloramines use reducing agents that neutralise the whole molecule — binding the ammonia as well as reducing the chlorine. Seachem Prime is the most widely used in the UK planted tank hobby; API Stress Coat and Tetra AquaSafe also handle chloramines when used at the correct dose. These products work instantly and are effective at very small volumes — a few millilitres treats hundreds of litres.

The important caveat: dose for the full volume of water you're adding, not the volume of the tank. Adding 100 litres of tap water to a 300-litre tank requires treatment calculated for 100 litres, not 300. Some aquarists treat the full tank volume regardless as a margin of safety — this is not harmful at normal doses.

RO and DI water

Reverse osmosis removes chlorine and chloramines by physical means — the membrane rejects them, and a carbon block pre-filter ahead of the membrane handles chlorine before it can damage the membrane itself (free chlorine degrades RO membranes over time). Water leaving an RO unit contains essentially zero chlorine or chloramine — no dechlorinator required.

The practical consideration is that RO water also contains essentially zero minerals, and cannot be used for most fish and plants without remineralisation. In practice, most hobbyists using RO mix it with treated tap water — which combines the mineral profile they want with the chloramine-free starting point that RO provides.

AquaCalc Dashboard

RO Mixer — build chloramine-free water

Mix RO/DI water with treated tap water at any ratio and see the resulting chemistry against target ranges for planted tanks and fish. Mixed TDS syncs to the pH Monitor automatically.

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What doesn't work

Ageing tap water — leaving it to stand for 24 hours — neutralises free chlorine through natural off-gassing. It does absolutely nothing to chloramines. Aeration speeds chlorine removal but has the same limitation. UV sterilisation at aquarium-level power outputs does not reliably break the chloramine bond. Activated carbon can reduce chloramine levels but is inconsistent, slow, and should not be relied upon as the primary treatment method for water changes.

If you don't know whether your water company uses chlorine or chloramines, the safest default is to assume chloramines and use a product that handles both. This costs nothing extra and removes any ambiguity.

How to find out what's in your tap water

Every UK water company publishes an annual water quality report, and most now offer a postcode-level lookup on their website. The figures you're looking for are free chlorine and total chlorine (or sometimes "combined chlorine").

If free chlorine and total chlorine are the same — or very close — your supplier is using chlorine only. If total chlorine is significantly higher than free chlorine, the difference is chloramines. A result showing "total chlorine 0.4 mg/L, free chlorine 0.1 mg/L" tells you that 0.3 mg/L of chloramine is present. That's the number your dechlorinator needs to handle.

You can verify this yourself with a DPD swimming pool test kit, which measures both free and total chlorine with different reagent tablets. DPD No.1 gives free chlorine; DPD No.3 gives total. The difference is combined chlorine — the chloramine fraction. These kits are inexpensive, widely available, and far more useful than the basic "chlorine" strips sold in aquarium shops, which typically measure only free chlorine.