The short answer
An ammonia spike happens when waste is produced faster than your filter bacteria can process it. The usual causes are overfeeding, overstocking, an uncycled or damaged filter, and dead matter rotting in the tank. In an established aquarium ammonia should always read zero, so any reading above that means something has overwhelmed or disrupted the biological filter. Fix the cause and dilute the ammonia with a water change.
The common triggers
- Overfeeding — uneaten food decays into ammonia. This is the number-one cause; feed only what fish clear in a couple of minutes. See our feeding guides.
- An uncycled filter — a brand-new tank hasn’t grown enough bacteria yet, so ammonia builds during the fish-in period. Learn how to cycle an aquarium first.
- Overstocking — too many fish produce more waste than the filter can keep up with.
- Dead matter — a dead fish, snail or rotting plant hidden in the substrate spikes ammonia fast.
- A disrupted filter — cleaning media in chlorinated tap water, or a power outage, can kill the bacteria. See why untreated tap water is a problem.
Tip: ammonia gets more toxic as pH and temperature rise, so the same reading is more dangerous in a warm, alkaline tank. If you see any ammonia, do a water change first and ask questions later.
How to respond
- Water change now — a 25–50% change with dechlorinated, temperature-matched water dilutes ammonia immediately. Follow how to do a water change.
- Stop feeding for a day or two to cut the input.
- Find and remove any dead matter or excess waste with a gravel cleaner.
- Support the filter with a bacteria starter so the colony rebuilds.
Confirm with a test
You can’t see ammonia, so a test kit is essential — it turns a vague worry into a number you can act on. Test daily until it holds at zero again, and see the water testing hub for how ammonia, nitrite and nitrate connect.