The short answer
Tap water contains chlorine or chloramine, added to kill bacteria in drinking water — which means it also harms your fish and your filter bacteria. The simple, reliable fix is a water conditioner (dechlorinator): add the dosed amount to your replacement water and it neutralises the chlorine instantly. Do this every single time you add tap water, whether for a water change or a top-up.
Why it matters
Chlorine damages fish gills and skin, and chloramine is toxic to fish and to the beneficial bacteria that run your cycle. Adding untreated tap water can stress or kill fish and set back your cycle — a common reason a tank won’t cycle. Even a small top-up adds chlorine, so treat all tap water.
Most water utilities now use chloramine (chlorine bonded to ammonia) rather than plain chlorine, because it’s more stable. That stability is the catch: it won’t evaporate on its own.
How to do it
- Get a water conditioner — a concentrated dechlorinator like Seachem Prime treats a large volume per capful. See our conditioner picks.
- Dose for your volume. Measure the dose for the amount of new water using the bottle’s instructions.
- Add it as you refill — either treat the new water in the bucket first, or dose the tank as the fresh water goes in.
- It works instantly — no waiting required. The chlorine is neutralised on contact.
Every time, no exceptions
Make conditioning automatic: every bucket of tap water, every top-up. A detoxifying conditioner also has the bonus of temporarily binding ammonia and nitrite, which helps during a cycle or a spike — see lowering ammonia fast. For whether your tap water is otherwise safe, see is tap water safe for fish, and for the routine, how to do a water change.