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

Ammonia is the invisible killer in aquariums. Fish produce it through their gills and waste, and it accumulates when bacteria haven't colonized your filter yet or when the tank is overstocked, overfed, or hasn't been cleaned in too long. Even 0.5 parts per million (ppm) of ammonia stresses fish; at 2 ppm or higher, most fish die within days. The good news is that ammonia drops fast once you know what to do.

The fastest way to lower ammonia right now is a partial water change—remove 25 to 50 percent of the water and replace it with dechlorinated water at the same temperature. This dilutes the ammonia when ready. If your ammonia test shows anything above 0.25 ppm, do a water change today. If it's above 1 ppm, do a 50 percent change and test again in a few hours.

Key Takeaways

  • A 25 to 50 percent water change dilutes ammonia within hours and is the fastest emergency fix.
  • Beneficial bacteria in the filter convert ammonia to nitrite, then to nitrate—this process takes two to four weeks in a new tank.
  • Overfeeding and overstocking are the most common causes; feed only what fish eat in two to three minutes, once or twice daily.
  • A filter rated for your tank size and run 24 hours a day is essential; bacteria need constant water flow to survive.
  • Test ammonia weekly until it stays at zero, then monthly once the tank is established.

Why ammonia appears in the first place

Fish produce ammonia constantly through their gills and in their waste. In a new tank with no established filter bacteria, ammonia has nowhere to go and builds up to toxic levels within days. In an established tank, ammonia usually means one of three things: the filter is too small, the tank is overstocked, or you're feeding too much.

Uneaten food and dead plants also decay into ammonia. If you drop in a pinch of food and your fish ignore half of it, that half rots on the bottom and becomes ammonia. A single dead fish left in the tank for a day can spike ammonia to dangerous levels. Check the substrate and decorations for hidden food and dead matter, and remove anything you find with a siphon or net.

How beneficial bacteria convert ammonia to less toxic compounds

The real solution to ammonia is nitrifying bacteria in your filter. These bacteria eat ammonia and convert it to nitrite, which is also toxic. A second type of bacteria then converts nitrite to nitrate, which is much less harmful and is removed during water changes. This whole process is called the nitrogen cycle.

In a brand-new tank, this cycle takes two to four weeks to establish. During that time, ammonia and nitrite will spike. In an established tank with a healthy filter, ammonia should stay at zero because the bacteria are already there and working. If ammonia is rising in an old tank, the bacteria are either dead (usually from a filter cleaning or power outage) or overwhelmed by too much waste.

You can speed up the cycle in a new tank by adding filter media or substrate from an established tank, or by using a bottled bacterial starter—these contain live nitrifying bacteria. However, even with these shortcuts, test the water every few days and do water changes if ammonia or nitrite spike above 0.5 ppm.

Stop overfeeding and overstocking when ready

Overfeeding is the number-one cause of ammonia problems in established tanks. Fish have small stomachs and don't need much food. Feed only what they will eat in two to three minutes, once or twice a day. If food is still floating or sinking uneaten after three minutes, you fed too much. Remove the excess with a net and feed less next time.

Overstocking—keeping too many fish in the tank—means too much waste and not enough filter capacity. A general rule is one inch of fish per gallon of water, though this varies by species and filter strength. A 20-gallon tank with a small filter should hold fewer fish than one with a large, powerful filter. If you have more fish than the tank can handle, move some to another tank or rehome them.

Make sure your filter is the right size and running constantly

Your filter must be rated for at least the volume of your tank, and ideally larger. A 20-gallon tank should have a filter rated for 20 gallons or more. The filter must run 24 hours a day, seven days a week—the bacteria inside die if the water stops flowing. If your filter is off, turn it on when ready. If it's clogged, clean it gently in old tank water (not tap water, which kills the bacteria) and put it back.

Never replace the entire filter media at once. If you need to clean it, rinse it in a bucket of water you've siphoned from the tank. This keeps the bacteria alive. If the filter is falling apart or broken, replace it with one rated for your tank size, and if possible, move some of the old media into the new filter to preserve the bacteria colony.

Do water changes weekly until ammonia stays at zero

Once you've done an emergency water change and fixed the cause (stopped overfeeding, removed dead fish, upgraded the filter), test ammonia every three to four days. If it's still above 0.25 ppm, do another 25 percent water change. Keep doing this until ammonia reads zero for two weeks in a row.

Once the tank is established and ammonia stays at zero, test it once a month. If it ever creeps back up, it's a sign that something has changed—you've added more fish, started feeding more, or the filter needs cleaning. Catch it early and do a water change before it becomes a crisis.

What to do if ammonia is spiking and you can't figure out why

If you've done water changes, stopped overfeeding, and the filter is clean and running, but ammonia is still rising, look for a hidden source. Check behind decorations and under the substrate for dead fish, uneaten food, or rotting plants. Siphon the gravel thoroughly to remove waste buildup. If the tank smells foul or the water looks cloudy, there's organic matter decomposing somewhere.

If the filter itself is the problem, test whether water is actually flowing through it. Hold your hand near the outlet—you should feel a steady current. If the flow is weak or stopped, the intake tube may be clogged or the motor may be failing. Clean the intake and outlet tubes first; if that doesn't help, the filter may need replacement.

In a new tank, ammonia spikes are normal. If you're in the first two weeks, do 25 percent water changes every three days and be patient. The bacteria will establish themselves, and ammonia will drop on its own once the cycle completes.

Frequently Asked Questions

How long does it take for ammonia to go down after a water change?

Ammonia levels drop within hours of a water change because you've diluted it directly. If you remove 50 percent of the water, ammonia is cut in half when ready. However, fish continue to produce ammonia, so levels will start rising again unless you've also fixed the underlying cause—overfeeding, overstocking, or a weak filter.

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

Products like ammonia-binding resins or zeolite can temporarily bind ammonia, but they don't solve the problem—they just hide it. Once the product is saturated, ammonia rises again. Water changes are faster, cheaper, and actually remove the ammonia from the tank. Use chemicals only as a temporary measure while you fix the real cause.

What ammonia level is safe for fish?

Zero ppm is the target. Any ammonia above 0.25 ppm causes stress and damage to fish gills over time. Most fish begin dying at 2 ppm or higher. If your test shows anything above zero, do a water change and investigate the cause.

Do I need to add bacteria to a new tank, or will it cycle on its own?

A new tank will cycle on its own in two to four weeks, but ammonia and nitrite will spike dangerously during that time. Bottled bacterial starters can speed this up, and moving filter media from an established tank is even faster. Either way, test the water every few days and do water changes if ammonia or nitrite spike above 0.5 ppm.

Why did ammonia suddenly spike in my established tank?

Something has changed. The most common causes are overfeeding, adding too many fish at once, a dead fish hidden in the tank, or the filter being turned off or clogged. Check all four. If the filter is running and clean, the tank isn't overstocked, and you haven't found a dead fish, you may be feeding more than you realize—cut the amount in half and test again in a few days.