Ammonia builds up when fish waste and uneaten food decay, and it poisons fish at levels above 0.25 parts per million (ppm)

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. Do this before trying anything else if your ammonia reading is above 1 ppm or your fish are gasping at the surface.

After the water change, the real work is stopping ammonia from building up again. Ammonia comes from two sources: fish waste (which contains ammonia directly) and the breakdown of uneaten food and dead plant matter. You control ammonia by removing waste before it decays and by letting beneficial bacteria convert ammonia into less toxic compounds.

Key Takeaways

  • A 25 to 50 percent water change removes ammonia when ready and is the first step when levels are high.
  • Beneficial bacteria in the filter convert ammonia to nitrite and then nitrate, but this takes weeks to establish in a new tank.
  • Overfeeding is the most common cause of ammonia buildup—feed only what fish finish in two to three minutes, once or twice daily.
  • A filter rated for at least twice your tank volume per hour keeps waste from accumulating faster than bacteria can process it.
  • Test ammonia weekly until it stays below 0.25 ppm consistently, then monthly after that.

Do a partial water change right away if ammonia is high

If your test kit shows ammonia above 0.5 ppm, or if fish are showing stress (gasping, clamped fins, lethargy), change 25 to 50 percent of the water when ready. Use water that has been treated with a dechlorinator if your tap water is chlorinated—chlorine kills the beneficial bacteria you need. Let the new water reach the same temperature as the tank before adding it, or use water within a few degrees of tank temperature to avoid shocking the fish.

Do not remove the filter media or clean the gravel during this water change. You want to keep the bacteria in the filter and substrate intact. Just siphon water from the top and middle of the tank, avoiding the gravel, and pour in the fresh water slowly.

Reduce feeding to stop ammonia from building up again

Overfeeding is the single most common cause of ammonia spikes. Uneaten food sinks and rots, releasing ammonia. Feed only what your fish will finish in two to three minutes, once or twice daily. Most home aquarists overfeed by a factor of two or three.

Remove any food that sinks to the bottom and is not eaten within five minutes. Use a small net or siphon to pull it out before it decays. If you are feeding flake food, pinch a small amount between your fingers—usually less than you think is necessary. Pellets are denser and easier to portion: a pinch or two per fish per feeding is a safe starting point.

Make sure your filter is large enough and running properly

A filter should process the entire tank volume at least twice per hour. A 20-gallon tank needs a filter rated for 40 gallons per hour (GPH) or more. If your filter is undersized, ammonia will accumulate faster than bacteria can process it, no matter how little you feed.

Check that water is flowing through the filter outlet at a steady rate. If flow has slowed, the filter media is clogged. Rinse the filter media in old tank water (not tap water, which kills bacteria) to clear debris without destroying the bacterial colony. Do this every two to four weeks, depending on how much waste accumulates.

Let beneficial bacteria establish in the filter and substrate

Beneficial bacteria convert ammonia into nitrite, and then other bacteria convert nitrite into nitrate. This process is called the nitrogen cycle, and it takes three to six weeks to establish in a new tank. During this time, ammonia and nitrite will spike even if you do everything right. Weekly water changes of 25 percent help keep levels from becoming toxic to fish while the bacteria colony grows.

Do not replace the filter media during this period. The bacteria live on the media surface. If you must clean it, rinse it gently in old tank water only. Once ammonia and nitrite both read zero and nitrate is present, the cycle is complete and ammonia should stay low as long as you do not overfeed or skip maintenance.

Test ammonia weekly until it stabilizes below 0.25 ppm

Use a liquid test kit rather than test strips—they are more accurate and cost less per test. Add the reagent drops to a sample of tank water according to the kit instructions, wait the full time (usually five minutes), and compare the color to the chart. Record the reading and the date.

If ammonia is above 0.25 ppm, do a 25 percent water change and check again in three days. If it is below 0.25 ppm, test again in one week. Once ammonia stays below 0.25 ppm for four weeks in a row, you can test monthly instead. If ammonia spikes again later, go back to weekly testing and look for the cause: overfeeding, a dead fish hidden in the tank, or a filter that has stopped working.

Remove dead fish and decaying plants when ready

A dead fish or rotting plant matter releases a large amount of ammonia as it decays. Check the tank daily for fish that are not moving or plants that are turning brown and mushy. Remove them with a net as soon as you spot them.

If you find a dead fish, do a 25 percent water change the same day. If the fish has been dead for more than a day, do a 50 percent change instead. Check ammonia the next day to see if it is still rising.

Frequently Asked Questions

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

Chemical ammonia removers (like Seachem Prime or similar products) bind ammonia so it is less toxic, but they do not remove it from the water. A water change actually removes ammonia. Chemicals are useful as a temporary measure while you do a water change, but they are not a substitute for it. Use them if ammonia is dangerously high and you cannot do a water change when ready.

How often should I clean the gravel if ammonia keeps spiking?

Gravel traps waste and decaying food, which release ammonia. Vacuum the gravel during your weekly water change, moving the siphon slowly across the bottom. This removes debris without destroying the bacteria living in the substrate. If ammonia is still spiking after you start vacuuming weekly, the problem is likely overfeeding or an undersized filter, not dirty gravel.

Will adding more plants lower ammonia?

Live plants consume ammonia as a nutrient, so they do help, but only slowly. Plants are not a substitute for water changes or a properly sized filter. They work best as part of a complete system: good feeding habits, adequate filtration, and regular maintenance. Fast-growing plants like water sprite or ludwigia help more than slow-growing ones.

What if ammonia is zero but nitrite is high?

This means the first stage of the nitrogen cycle is working (ammonia is being converted to nitrite) but the second stage is not yet established. Continue weekly 25 percent water changes and be patient. Nitrite is toxic like ammonia, so keep it below 0.25 ppm the same way. The cycle will complete in another week or two.

Can I use tap water directly, or do I have to treat it?

If your tap water is chlorinated, you must treat it with a dechlorinator before adding it to the tank. Chlorine kills beneficial bacteria. If your tap water is not chlorinated (some municipal systems do not use it, and well water typically is not), you can use it directly. Check with your water provider if you are unsure. Dechlorinator is inexpensive and lasts a long time, so it is safer to treat it than to guess.