Ammonia is the most immediately dangerous substance in an aquarium — even 0.25 ppm damages fish gills, and higher levels kill within hours. If your test shows any ammonia at all, act now.
Here is the fast fix, followed by how to stop it coming back.
- Any ammonia reading above 0 is an emergency — do an immediate 50% water change.
- Dose Seachem Prime at 5× strength to temporarily detoxify ammonia while you fix the cause.
- Stop feeding for 48 hours and add aeration to help stressed fish.
- Long term, cycle the tank fully, feed less, avoid overstocking, and never rinse filter media in tap water.
Immediate action (do this now)
1. Do a large water change. Change 50% of the water with dechlorinated water at matching temperature. This is the single fastest way to dilute ammonia.
2. Dose Seachem Prime at 5× the normal amount. Prime temporarily detoxifies up to 1 ppm of ammonia for 24–48 hours, buying you time.
3. Stop feeding for 48 hours. Less food in means less ammonia out.
4. Add aeration. Ammonia-stressed fish need more oxygen — add an air stone.
5. Check for a dead fish or rotting food behind decor — a common hidden ammonia source.
Why ammonia rises
The most common causes are: a tank that is not yet cycled, overfeeding, overstocking, a dead fish, or rinsing filter media in tap water (which kills the bacteria that consume ammonia).
Stopping it long term
- Make sure the tank is fully cycled before stocking — see our cycling guide.
- Feed only what fish clear in two minutes, twice a day.
- Never rinse filter media in tap water — use old tank water.
- Do not overstock. See stocking guidance.
- Test weekly with an API Master Test Kit.
Frequently asked questions
How do I lower ammonia in my fish tank quickly?
Do a 50% water change with dechlorinated water, dose Seachem Prime at 5× strength to detoxify remaining ammonia, stop feeding for 48 hours, and add an air stone. Then find and fix the underlying cause.
What causes high ammonia in an aquarium?
An uncycled tank, overfeeding, overstocking, a dead fish or rotting food, or rinsing filter media in tap water (which kills the beneficial bacteria that remove ammonia).
Is ammonia remover safe for fish?
Products like Seachem Prime safely detoxify ammonia temporarily and are ideal in emergencies. They do not replace water changes and fixing the underlying cause, but they protect fish in the meantime.
— Jaqline Smalling, FishForever
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