Dechlorinator 101: Why Tap Water Can Kill Fish

Quick answer

Make tap water safe for fish before every fill, top-up, and water change.

Tap water is treated with chlorine and often chloramine so people can drink it. Those same chemicals burn fish gills and kill beneficial filter bacteria. Always dose a quality dechlorinator (water conditioner) for the full volume of new water before it enters the tank. Match the product to your water type: many city supplies use chloramine, which needs a conditioner that handles both chlorine and chloramine, not chlorine alone. When in doubt, treat every drop.

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Why it matters

Municipal water is disinfected for human safety. Chlorine and chloramine are oxidisers that damage delicate gill tissue and can collapse the nitrogen cycle by killing nitrifying bacteria in the filter. A water change that should improve water quality can become a poisoning event if the new water is raw from the tap.

This is not an advanced-only step. It is baseline fishkeeping, the same class of habit as not washing the filter media under hot tap water.

What good looks like

You keep a bottle of conditioner next to the bucket. Every water change uses measured doses, temperature-matched water, and a calm pour or airline siphon return. Fish stay colour-normal and do not rocket to the surface gasping after you finish. The filter keeps processing ammonia after large changes because you did not bleach the bacteria.

Measure conditioner for the bucket volume you are about to add, not for the whole tank unless the label says so.

How dechlorinators work

Most modern conditioners:

• Neutralise free chlorine quickly
• Break chloramine bonds and bind the ammonia that is released (important: chloramine = chlorine + ammonia)
• Often bind some heavy metals from pipes
• Sometimes add slime-coat support (marketing varies; the dechlorination is the non-negotiable part)

Read the front label for chloramine. If your utility uses chloramine, a chlorine-only product is the wrong tool.

How to dose correctly

1. Know your change volume (example: 40 L from a 100 L tank is a 40% change).
2. Fill a clean bucket with tap water.
3. Dose conditioner for that bucket volume per label.
4. Match temperature to the tank within about 1°C if possible (2°F-ish).
5. Add water slowly to avoid shocking fish and stirring mulm into a cloud.

Pour conditioned water back slowly during a water change.

Special situations

• Chloramine cities: Confirm on the utility website or bill insert. Use a chloramine-capable conditioner.
• Well water: May lack chlorine but can have metals, hardness swings, or hydrogen sulfide; test rather than assuming "natural = safe".
• RO/DI water: No disinfectant, but also no minerals; remineralise for most community fish and shrimp before use.
• Medication periods: Some meds interact with conditioners or carbon; follow both labels.
• Emergency tap dump: If someone poured untreated water in already, dose the whole tank for the full volume immediately per label, add air, and watch fish.

Boiling can reduce free chlorine but does not reliably fix chloramine and does nothing useful for a practical water-change workflow. Use conditioner.

Living context by region

• Hong Kong / Singapore / many dense cities: Municipal chloramine or chlorine is common; always condition.
• EU / UK: Utility disinfectant varies by region; check local water report.
• US: Large swaths use chloramine; "let it sit overnight" is not enough.
• Rural wells: Test; do not skip parameter checks because there is no city chlorine.
• Currency/cost: Conditioner is cheap insurance versus livestock loss in any currency (HK$, US$, €, ¥, ₩…).

Common mistakes

• Letting water "sit out" and assuming chloramine is gone
• Dosing for the whole tank when only adding a bucket (or the reverse of the label)
• Hot tap water for changes (metals, temperature shock)
• Rinsing bio-media under untreated tap
• Skipping top-ups on nano tanks
• Using garden hoses that leach or hold stagnant water without flushing

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