Dechlorinator 101: Why Tap Water Can Kill Fish
Tap water is treated with chlorine or chloramine to make it safe for people — but those same chemicals burn fish gills and wipe out the good bacteria in your filter. This guide explains what a dechlorinator does, how to dose it correctly, and how to add new water safely at every water change.
Quick answer
Almost all municipal tap water contains chlorine or chloramine to kill microbes for human safety. Both are toxic to fish and to the beneficial bacteria in your filter. Always treat new tap water with a dechlorinator (water conditioner) at the correct dose before it reaches your fish — every time you fill the tank or do a water change.
Tap water is treated with chlorine or chloramine to make it safe for people — but those same chemicals burn fish gills and wipe out the good bacteria in your filter.
What is actually in tap water
Water utilities disinfect supply with either chlorine or chloramine (chlorine bonded to ammonia). Chlorine damages the delicate gill tissue fish use to breathe. Chloramine is worse for aquariums because it is more stable and also releases ammonia when broken down. Both also kill the nitrifying bacteria living in your filter — the invisible colony that keeps your water safe. Lose that colony and the tank can crash days later.
What a dechlorinator does
Modern water conditioners chemically neutralise chlorine and break the chloramine bond, then bind the leftover ammonia into a form your filter bacteria can safely process. Better conditioners also detoxify small amounts of heavy metals from old pipes. The reaction is fast, so there is no need to pre-mix for hours — treated water is safe to use within a minute or two.
Dose the conditioner for the full tank volume before the new water goes in.
How to dose correctly
Read your specific bottle; strengths vary and concentrated brands use tiny amounts. The key rule: dose for the volume of the whole aquarium, not only the fresh water you are adding, unless the label says otherwise. Slight overdosing within the label's stated safety margin is generally harmless; badly under-dosing leaves live chlorine in the tank. Use a proper measuring cap, not a guess.
Adding water safely at a water change
Match the new water's temperature to the tank by feel or thermometer before adding it — a cold blast is a stressor on its own. Pour slowly against the glass or onto a plate to avoid blasting the substrate. In Hong Kong and Taiwan high-rise flats, tap water can run warm in summer and cool in winter, so check the temperature each season rather than assuming.
Add treated water slowly so temperature and chemistry stay stable.
Special situations
If you use an inline python-style water changer that refills straight from the tap, dose the whole tank's worth of conditioner into the aquarium just before or as you refill, and refill slowly. Well water and rainwater have different risks (no chlorine, but possible contaminants or zero mineral content) and may need testing. If your building recently flushed or worked on the pipes, expect a temporary spike in chlorine.
Quick FAQs
Can I just let tap water sit out overnight instead? Only if your supply uses plain chlorine, and even then it is slow and unreliable. It does not remove chloramine at all, so a conditioner is the safe choice.
Will a conditioner remove ammonia from my cycle? It temporarily detoxifies ammonia from chloramine, but it is not a substitute for a working filter or water changes. Fix the underlying cause.
Can I overdose the conditioner? Most brands have a wide safety margin, and 1-2x the dose is usually fine. Very large overdoses can lower oxygen, so measure sensibly.
Do I need it for a top-up to replace evaporation? Evaporation leaves minerals behind, so top-ups are small — but any tap water added should still be treated to be safe.