Ammonia in a fish tank: causes and how to lower it

Ammonia is the most common reason fish get sick in a home aquarium. In a healthy tank it should always read zero. Here's where it comes from, how to bring it down fast, and how to keep it at zero.

Where ammonia comes from

Fish give off ammonia as waste, and rotting food and plants make more. In a healthy tank, good bacteria living in the filter turn ammonia into nitrite, and then into nitrate, which is much less harmful and gets removed by water changes. This is called the nitrogen cycle.

If you can measure ammonia, more is being made than the bacteria can handle.

How much is too much?

Aim for 0 ppm. Any reading above zero in an established tank means something is wrong. Ammonia is more harmful in warm water and when the pH is above 7, so the same reading can be more dangerous in one tank than another. Don't wait to see if it gets worse.

Signs in your fish: gasping at the surface, red or inflamed gills, sitting on the bottom, clamped fins and not eating.

Common causes

• A new tank that isn't cycled yet. The bacteria haven't grown yet.
• Too many fish, or too many added at once. The bacteria need time to catch up. Add fish a few at a time.
• Overfeeding. Uneaten food rots on the bottom.
• Something dead in the tank. A missing fish or snail, or rotting plant leaves.
• A filter that was cleaned too hard. Rinsing media under the tap, or replacing all of it at once, throws away the bacteria. See how to clean a filter.
• A filter that stopped for hours, after a power cut or a blocked intake.
• Some medicines, especially antibiotics, can harm the filter bacteria.
• Your tap water. Water treated with chloramine can show ammonia. Test a glass of tap water to check.

How to lower ammonia fast

1. Do a 30–50% water change with conditioned water at the same temperature. Repeat every day until ammonia reads zero, or very close.
2. Find and remove the source. Look for dead fish or snails and siphon out uneaten food.
3. Feed very lightly, or skip a day or two. Less food means less ammonia.
4. Use a conditioner that detoxifies ammonia, such as Seachem Prime, following the bottle. It makes ammonia less harmful for about a day, which buys time, but it doesn't remove it. Your test kit may still show ammonia while it's working.
5. Add bottled bacteria to help the filter catch up.
6. Leave the filter alone. Don't clean or replace filter media until things settle.
7. Test every day until ammonia and nitrite read zero for a week.

What about nitrite?

Nitrite is the next step in the cycle and is just as harmful. If nitrite is above zero, use the same fixes: water changes, light feeding and patience while the bacteria catch up.

Related: How to cycle a new fish tank · Why are my fish dying? · How to do a water change