Ammonia spike in a fish tank: how to fix it fast (new tank syndrome guide)

Quick answer

If ammonia or nitrite is above 0, act immediately: do a 25% to 50% water change, add dechlorinator, increase aeration, and stop overfeeding. Keep the filter running, do not replace filter media, and test daily until readings return to 0. Most spikes come from adding too many fish too fast, overfeeding, or disrupting the filter bacteria.

The emergency plan (do this today)

1. Water change: 25% to 50% (match temperature closely)
2. Dechlorinate properly: treat the new water every time
3. Add oxygen: point the filter output to ripple the surface or add an air stone
4. Stop feeding for 24 hours: then feed very lightly
5. Test again in a few hours and again the next day

If fish are gasping at the surface or acting distressed, prioritize aeration and water changes.

What NOT to do

• Do not do a 100% water change unless you have a specific reason.
• Do not replace all filter media.
• Do not rinse filter media under tap water.
• Do not add more fish.

Those actions often reset the cycle and make the problem repeat.

Why ammonia spikes happen

Common triggers:

• Adding fish before the tank is cycled
• Adding too many fish at once after cycling
• Overfeeding or leaving food to rot
• Cleaning the tank “too well” and removing beneficial bacteria
• A filter that stopped overnight or lost flow

Stabilize the tank over the next 7 days

• Test ammonia and nitrite daily.
• Do partial water changes as needed to keep ammonia and nitrite at 0.
• Feed lightly until stability returns.
• If you have space, add easy live plants for extra buffer.

Prevention checklist

• Add fish in stages (wait 1 to 2 weeks between additions).
• Keep a consistent water-change routine.
• Clean filter media gently in removed tank water.
• Do not rely on “quick fix” chemicals instead of testing.

FAQ

Is bottled bacteria a solution?

It can help, but it does not replace testing and good maintenance. Always confirm readings return to 0.

Should I use ammonia remover products?

They can reduce harm in emergencies, but you still need the underlying biological filter to handle waste long-term.

How long until things are stable again?

Often a few days to a couple of weeks, depending on what caused the spike and how stocked the tank is.

Next step

Want a beginner-friendly fish recommendation that fits a low-risk setup?