Ammonia builds up when fish waste and uneaten food decay, and it poisons fish even at low concentrations

Ammonia is a toxic compound that forms from fish waste, decaying plants, and leftover food sitting on the tank bottom. Fish produce ammonia through their gills constantly, and it accumulates fastest in tanks that are overstocked, overfed, or have poor water circulation. Even small amounts—as little as 0.5 parts per million (ppm)—stress fish and damage their gills over time. You can measure ammonia with a test kit, and the reading should be zero or undetectable in an established tank.

The fastest way to lower ammonia is a partial water change: remove 25 to 50 percent of the tank water and replace it with fresh, dechlorinated water. This dilutes the ammonia when ready. For a permanent fix, you need to reduce what creates ammonia in the first place—overfeeding and overstocking—and may support your filter is working and your tank has had time to cycle.

Key Takeaways

  • A 25 to 50 percent water change removes ammonia right away and is the first step when levels are high.
  • Feed fish only what they can eat in two to three minutes, once or twice daily, because uneaten food decays and produces ammonia.
  • Stock the tank according to the one-inch-per-gallon rule as a starting point: one inch of fish body length per gallon of water.
  • A filter with an active biological colony (bacteria that consume ammonia) takes two to four weeks to establish in a new tank.
  • Siphon the gravel weekly to remove settled waste, which is a major source of ammonia production.

Do a Partial Water Change Right Away

Remove 25 to 50 percent of the tank water using a siphon or aquarium vacuum. A siphon is a tube: place one end in the tank, create suction by mouth or by filling the tube with water first, and let gravity pull water into a bucket below the tank level. An aquarium vacuum is a wider tube with a pump handle that sucks water and gravel at the same time. Either way, the water you remove carries dissolved ammonia with it.

Replace the removed water with fresh water that has been treated with a dechlorinator if your tap water is chlorinated. Most municipal water supplies are, and chlorine kills the beneficial bacteria in your filter. Let the new water reach the same temperature as the tank water before adding it—a sudden temperature drop stresses fish. This water change is temporary relief; ammonia will build up again if you do not address the source.

Feed Less and Less Frequently

Overfeeding is the single largest cause of ammonia spikes in home aquariums. Feed your fish only what they can consume in two to three minutes, once or twice daily. Most home aquarists feed too much because fish always look hungry—they beg at the glass because they see movement, not because they are starving. Uneaten food sinks to the bottom, where bacteria break it down and produce ammonia.

Different fish have different needs. Goldfish and plecos are heavy eaters and need more food than tetras or guppies. Start with a small pinch, watch the fish eat, and stop when food begins to fall uneaten to the bottom. If you miss a feeding or two, your fish will be fine; they can go several days without food. If you overfeed every day, ammonia will spike every week.

Remove Uneaten Food and Settled Waste

Use an aquarium vacuum or a thin siphon to clean the gravel once a week. Push the vacuum tube into the gravel in one corner and let it pull up water, gravel, and the debris settled between the stones. The gravel will fall back into the tank while the waste and water go into your bucket. This removes a major source of ammonia before it dissolves into the water column.

If you see food floating after feeding time, remove it with a net before it sinks. Dead plants and dead fish also decay and produce ammonia, so remove them as soon as you notice them. A tank with good maintenance—weekly vacuuming and daily observation—will have much lower ammonia than one that is cleaned once a month.

Check That Your Filter Is Running and Sized Correctly

Your filter must run 24 hours a day. The motor should be audible or you should see water flowing through the output tube. If the filter is off or clogged, ammonia has nowhere to go. A clogged filter means the media (sponge, cartridge, or floss) is packed with waste and water cannot flow through it. Rinse the filter media in old tank water—never tap water, which kills the bacteria—and reinstall it.

The filter should be sized for your tank volume. A rule of thumb is that the filter should turn over the entire tank volume at least four times per hour. A 20-gallon tank needs a filter rated for at least 80 gallons per hour (GPH). Check your filter's GPH rating on the box or manual. If your filter is undersized, ammonia will build up faster than the bacteria can break it down, and you may need to upgrade.

Reduce the Number of Fish in the Tank

Overstocking is the second-largest cause of ammonia problems. A common guideline is the one-inch-per-gallon rule: one inch of fish body length per gallon of water. A 20-gallon tank can hold about 20 inches of fish—for example, four 5-inch fish or ten 2-inch fish. This is a starting point, not a hard limit, because some fish produce more waste than others. Goldfish and plecos are heavy waste producers and need more space than tetras.

If your tank is overstocked, you have two choices: move some fish to another tank or upgrade to a larger tank. Moving fish is faster and cheaper. If you cannot move them, do larger and more frequent water changes—50 percent twice a week instead of once a week—until you can reduce the population. Overstocking combined with overfeeding creates ammonia spikes that no filter can handle.

Wait for Your Filter to Cycle if the Tank Is New

A new tank has no bacteria to break down ammonia. Ammonia-eating bacteria (Nitrosomonas) and nitrite-eating bacteria (Nitrobacter) must grow in the filter media over two to four weeks. During this time, ammonia and nitrite will spike even if you do everything else right. This is called the nitrogen cycle.

To speed up cycling, add a bacterial starter culture (sold as "beneficial bacteria" or "cycle starter" at aquarium stores) or move filter media from an established tank into your new filter. You can also add a few hardy fish and do daily water changes until ammonia drops to zero. Once ammonia stays at zero for a week, the cycle is complete and your filter can handle the bioload of your fish.

Frequently Asked Questions

How often should I test ammonia?

Test ammonia twice a week in a new tank until the cycle is complete, then once a week in an established tank. If you notice fish gasping at the surface or acting lethargic, test when ready—these are signs of ammonia poisoning. Use a liquid test kit rather than strips; strips are less accurate.

Can I use ammonia-removing chemicals instead of water changes?

Chemicals like activated carbon or ammonia-binding resins remove ammonia temporarily, but they do not address the source. A water change removes ammonia and dilutes other waste at the same time. Use chemicals only as a short-term fix while you fix the real problem—overfeeding, overstocking, or a filter that is not running.

What if ammonia stays high even after I do all this?

Check that your filter is actually running and that the media is not clogged. Count your fish and compare to the one-inch-per-gallon rule. Watch yourself feed for a few days to make sure you are not overfeeding out of habit. If all three are correct and ammonia is still high, your tank may be too small for the fish you have, and you will need a larger tank or fewer fish.

Is ammonia the same as nitrite and nitrate?

No. Ammonia is produced by fish waste. Bacteria convert ammonia to nitrite, which is also toxic. Other bacteria convert nitrite to nitrate, which is less toxic but still builds up over time. All three should be tested, but ammonia and nitrite should be zero in an established tank.