The short answer
Ammonia is highly toxic to fish, so act quickly. The fastest safe way to lower it is a large water change (around 50%) with dechlorinated, temperature-matched water, followed by a dose of a detoxifying conditioner such as Seachem Prime to bind what remains. Then find and fix whatever caused the spike — because it will keep returning until you do.
Step 1: dilute it with a water change
A water change is the most direct way to physically remove ammonia. Change 50% now, and repeat over the next day or two if the reading stays up. Match temperature and dechlorinate the new water so you don’t add more stress. See how to do a water change.
Step 2: detoxify what’s left
A conditioner that detoxifies ammonia (not just dechlorinates) converts the remaining ammonia into a form fish can tolerate for a day or so, without stopping your filter bacteria from processing it. This buys crucial time. Browse our conditioner and bacteria picks.
Step 3: fix the cause
Water changes treat the symptom. Ammonia spikes come from:
- An uncycled or newly set-up tank — see what causes an ammonia spike.
- Overstocking or overfeeding overwhelming the filter.
- A dead fish, uneaten food or rotting plant — find and remove it.
- A crashed filter — a cleaned-too-hard or unpowered filter loses its bacteria.
The lasting fix
If the tank isn’t fully cycled, you’re in a fish-in cycle: keep testing, keep doing changes and dosing conditioner until your filter grows enough bacteria to hold ammonia at zero. Adding a bacteria starter speeds this up. For the full method, see how to cycle an aquarium and browse the filter hub.