Nitrites come from fish waste and uneaten food, and you lower them by doing partial water changes, cleaning the substrate, and letting beneficial bacteria establish themselves

Nitrites are toxic compounds that build up in aquariums as fish waste breaks down. They sit between ammonia (the first waste product) and nitrates (the final, less harmful form) in the nitrogen cycle. A reading above 0.25 ppm (parts per million) is dangerous to fish; anything above 1 ppm can kill them quickly. The fastest way to bring nitrites down is a 25 to 50 percent water change, which dilutes the nitrites when ready. After that, the real work is establishing the bacteria that convert nitrites into nitrates—a process that takes weeks in a new tank but should be automatic in an established one.

The reason nitrites spike is usually straightforward: you are overfeeding, the filter is clogged, or the tank is too new and the bacteria have not colonized yet. Once you understand which one is happening in your tank, the fix is straightforward.

Key Takeaways

  • A partial water change of 25 to 50 percent brings nitrites down within hours and is the first step in any spike.
  • Beneficial bacteria in the filter and substrate convert nitrites to nitrates, but they need time to grow and cannot be rushed.
  • Overfeeding and decaying plants or fish are the most common causes of nitrite spikes in established tanks.
  • In a new tank, nitrites will spike before they fall—this is normal and called the nitrogen cycle, and it takes three to six weeks.
  • If nitrites stay high after water changes and the tank has been running for months, the filter may be clogged or the bioload may be too heavy for the tank size.

Do a partial water change right away

Remove 25 to 50 percent of the water and replace it with fresh, dechlorinated water at the same temperature. This dilutes the nitrites in the tank when ready and gives you breathing room while you address the cause. Use a siphon or gravel vacuum to pull water from the bottom of the tank, where waste accumulates—this also removes some of the decaying material that is producing the nitrites in the first place.

Do not do a 100 percent water change. Removing all the water kills the beneficial bacteria living in the filter and substrate, which are the only things that will prevent nitrites from spiking again. A partial change removes the problem without destroying the solution. If nitrites are extremely high (above 2 ppm), you can do a 50 percent change every day for a few days, but return to normal maintenance once they drop below 1 ppm.

Clean the substrate and filter without destroying bacteria

Uneaten food and fish waste settle on the bottom and in the filter, where they rot and produce ammonia and nitrites. Use a gravel vacuum on the substrate during your water change—push it down into the gravel and let the siphon pull out debris without removing the gravel itself. Pay special attention to corners and under decorations where waste collects. You should see brown or black sludge come up; that is the waste you are removing.

For the filter, rinse the filter media (sponge, cartridge, or floss) in a bucket of water you removed from the tank. Use only that old tank water, never tap water, because chlorine and temperature changes kill the bacteria you need. Squeeze the media gently until the water runs clear. If the filter is clogged with brown sludge, it cannot move water through properly, and waste backs up in the tank. A clogged filter is one of the most common reasons nitrites spike in tanks that have been running for months.

Stop overfeeding and remove dead plants or fish

Fish produce waste in proportion to how much they eat. Uneaten food rots and produces ammonia, which bacteria convert to nitrites. Feed only what the fish will finish in two to three minutes, once or twice a day. Most aquarium fish are overfed, and reducing food is often enough to stop nitrite spikes on its own. If you have been feeding a pinch or a full spoon, try half that amount and watch whether the fish finish it.

Dead or decaying plants release organic matter that feeds the bacteria producing nitrites. Remove any leaves that are brown or falling apart. If a fish has died, remove it when ready—a dead fish decays quickly and creates a sudden ammonia and nitrite spike. Check the tank daily for uneaten food and remove it with a net or siphon before it sinks and rots.

Understand the nitrogen cycle in new tanks

In a brand-new aquarium, nitrites will spike sharply around day 7 to 10, peak around day 14 to 21, and then fall to zero by week 4 to 6. This is the nitrogen cycle, and it is normal. Ammonia-eating bacteria colonize the filter first, turning fish waste into nitrites. Then nitrite-eating bacteria colonize, turning nitrites into nitrates. Until the second group is established, nitrites climb. This happens even if you do everything right.

During this period, do 25 percent water changes every two to three days to keep nitrites below 1 ppm, which keeps fish alive while the cycle completes. Do not add fish all at once—add a few hardy fish first, let them produce waste to feed the bacteria, and add more fish gradually as the cycle progresses. Do not clean the filter during the cycle; you will kill the bacteria you are trying to grow. The cycle cannot be rushed with chemicals or additives, no matter what the bottle says.

Test water regularly to track progress

Buy a liquid test kit that measures ammonia, nitrite, and nitrate. Strips are cheaper but less accurate; a liquid kit costs more upfront but gives reliable readings. Test the water every two to three days while nitrites are high, and every week once they are stable. Write down the readings so you can see whether they are trending down or staying flat.

In an established tank, nitrites should always be 0 ppm. If they are above 0.25 ppm, something has changed—the filter is clogged, you are overfeeding, or there is a dead fish or plant you have not found. In a new tank, track the spike and fall to know when the cycle is complete and it is safe to add more fish. Testing takes five minutes and tells you whether your fixes are working.

Check whether your tank is overstocked or the filter is too small

A tank that is too small for the number of fish, or a filter that is too weak, cannot handle the bioload (the amount of waste the fish produce). If nitrites stay high even after water changes, cleaning, and reducing food, the tank itself may be the problem. A general rule is one inch of fish per gallon of water, though this varies by species and how much the fish eat. Some fish are heavy waste producers and need more space than others.

If the tank is overstocked, you have three options: remove some fish, upgrade to a larger tank, or upgrade to a stronger filter. A filter should process the entire tank volume at least three times per hour—a 20-gallon tank needs a filter rated for 60 gallons per hour or more. Check your filter's rating on the box or manual. If you are not sure whether your tank is overstocked, ask at a local aquarium store or describe your setup online; experienced aquarists can usually spot the problem quickly.

Frequently Asked Questions

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

A partial water change brings nitrites down within a few hours by dilution alone. However, the nitrites that remain will climb again unless the bacteria that convert them to nitrates are working. In an established tank with healthy bacteria, nitrites should return to zero within a few days. In a new tank during the nitrogen cycle, they will climb again after the water change.

Can I use chemicals to lower nitrites faster?

Products that claim to speed up the nitrogen cycle or remove nitrites exist, but they are not necessary and do not replace water changes and bacteria. The only reliable way to lower nitrites is water changes (when ready) and beneficial bacteria (over time). Focus on the basics: partial water changes, cleaning waste, and not overfeeding.

What if nitrites are high but ammonia is zero?

This means the ammonia-eating bacteria are working, but the nitrite-eating bacteria are not yet established or are overwhelmed. This is normal in a new tank and happens in established tanks when something kills the bacteria (like a filter cleaning with tap water or a sudden temperature drop). Continue water changes and do not clean the filter; the nitrite-eating bacteria will grow.

Should I do water changes every day if nitrites are very high?

If nitrites are above 2 ppm, a 50 percent water change every day is reasonable until they drop below 1 ppm. After that, return to every two to three days. Daily changes can stress fish and waste your time if the underlying cause (overfeeding, overstocking, clogged filter) is not fixed.

Can I add more fish while nitrites are high?

No. High nitrites are toxic, and adding fish adds more waste, which raises nitrites further. Wait until nitrites are zero and have stayed zero for at least a week before adding new fish. In a new tank, this means waiting for the nitrogen cycle to complete.