The short answer
You don’t actually need to let tap water sit at all — a water conditioner makes it safe in seconds. The old rule of leaving water out for 24 hours only helps plain chlorine evaporate, and even that is slow and unreliable. It does nothing for chloramine, which is stable and won’t gas off no matter how long you wait. A dechlorinator is faster, more dependable, and handles both.
Why “letting it sit” is outdated advice
Years ago, when supplies used only chlorine, standing water uncovered for a day let some of it escape into the air. But two things make this a poor plan today:
- Many water utilities now use chloramine, which stays in the water indefinitely.
- Even with plain chlorine, evaporation is slow and you can’t tell when it’s actually gone.
A conditioner neutralises chlorine and chloramine instantly, so there’s no guesswork. See is tap water safe for aquarium fish? for the full picture, and our conditioner picks.
What to do instead
- Add a water conditioner to new water for the volume you’re adding — or dose the tank as you refill.
- Match the temperature so fish aren’t shocked.
- Cycle the tank first. This is the real waiting period. A new tank needs to establish its biological filter before it can hold fish — read how to cycle an aquarium and use a test kit to confirm ammonia and nitrite read zero.
The bottom line
Skip the 24-hour myth. Treat the water with a conditioner and it’s safe from chlorine immediately. The genuine wait before adding fish is the cycling period — check the water testing hub to know exactly when the tank is ready, then follow good water change habits from there.