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A high ammonia, nitrite or nitrate reading is one of the most common reasons fish become stressed, ill or die. The good news is that all three can be brought down quickly and safely once you know which one you're dealing with and why it has risen.
This guide is for established freshwater tanks. If your aquarium is less than eight weeks old, read our guide on new tank syndrome and how to cycle an aquarium as well.
The nitrogen cycle in one minute
Fish waste and uneaten food break down into ammonia, which is highly toxic. Beneficial bacteria in your filter convert it into nitrite, which is also toxic. A second group of bacteria converts nitrite into nitrate, which is far less harmful but builds up over time and is removed by water changes and plants. When this chain breaks, or is overloaded, the toxic compounds build up.
Safe levels at a glance
| Parameter | Target | Act now if |
|---|---|---|
| Ammonia (NH3/NH4) | 0 ppm | Any reading above 0.25 ppm |
| Nitrite (NO2) | 0 ppm | Any reading above 0.25 ppm |
| Nitrate (NO3) | Below 20–40 ppm | Above 50 ppm, or lower for sensitive species and shrimp |
Ammonia becomes much more toxic as pH and temperature rise, so a reading of 0.5 ppm in a warm tank at pH 8 is more dangerous than the same reading at pH 6.5.
Test properly first
Liquid test kits are more accurate than strips for diagnosing a problem. The API Freshwater Master Test Kit covers ammonia, nitrite, nitrate and pH in one box, or you can buy single tests such as the NT Labs Ammonia, Nitrite and Nitrate kits. A constant-reading monitor such as the Seachem Ammonia Alert gives an early warning between tests. Browse all test kits.
Test your tap water too. Some UK supplies contain measurable nitrate, and some contain chloramine, which can show as ammonia once it has been treated.
How to lower ammonia
Common causes
- Overfeeding or uneaten food rotting in the gravel.
- A dead fish, snail or decaying plant hidden in the tank.
- Cleaning the filter in tap water, or replacing all the media at once, which kills the bacteria.
- Too many fish added at once, or the tank is simply overstocked.
- Medication that has harmed the filter bacteria.
What to do
- Do a 25–50% water change with temperature-matched, dechlorinated water. Repeat daily until ammonia is below 0.25 ppm.
- Detoxify while the filter catches up. Seachem Prime dechlorinates tap water and binds ammonia and nitrite into a less toxic form for around 24–48 hours, so it protects fish between water changes.
- Stop or cut feeding for a few days. Fish come to no harm, and you'll stop adding fuel to the problem.
- Find and remove the source. Look behind decor and under plants for anything dead or decaying, and gravel-clean the substrate.
- Boost the bacteria. Add a live bacteria product such as Seachem Stability, NT Labs Satus or API Quick Start. See our nitrifying bacteria range.
How to lower nitrite
Nitrite usually follows an ammonia spike, because the second group of bacteria grows more slowly. Fish with nitrite poisoning often gasp at the surface and have brownish gills. The fixes are the same as for ammonia: water changes, Prime to detoxify, reduced feeding and added bacteria. Keep testing daily; nitrite often peaks a few days after ammonia falls.
Adding a small amount of aquarium salt (sodium chloride) protects fish from nitrite uptake, but check that all your species tolerate salt, and don't use it with plants and scaleless fish that are sensitive.
How to lower nitrate
Nitrate builds up gradually in every tank. High nitrate causes stress, poor colour, stunted growth and is a major driver of algae.
- Increase water changes. Change 20–30% weekly. If nitrate stays high, change more often rather than in one huge change.
- Clean the substrate and filter regularly. Trapped waste turns into nitrate. A siphon such as the Fluval ProVac Powered Gravel Cleaner or Eheim Gravel Cleaner makes this quicker. Browse all gravel cleaners.
- Add fast-growing plants. Stem plants and floating plants absorb nitrate directly. See our live aquarium plants.
- Use nitrate-removing media. Seachem de*Nitrate and Seachem Purigen help reduce nitrate and the organic waste that becomes nitrate. Media helps, but it doesn't replace water changes.
- Check your tap water. If it already contains 30–40 ppm nitrate, you'll struggle to get below that with tap water alone. Mixing in water from a reverse osmosis unit solves this.
How to stop it happening again
- Feed only what your fish eat in two to three minutes.
- Rinse filter media in old tank water, never tap water, and never replace all of it at once.
- Add new fish a few at a time so the bacteria can keep up.
- Give the filter extra biological capacity with a high-surface media such as Seachem Matrix. See all filter media.
- Test weekly and keep a simple log, so you spot a trend before your fish do.
For a full routine, read our water change guide and browse our tap water conditioners. If your readings won't come down, our team is happy to look at your test results with you.
