Whether you keep a freshwater or a saltwater aquarium, a high concentration of nitrates in the tank can quietly cause problems. We will look at what high nitrate levels can do to your fish, what a safe level is, and how to lower nitrates in a fish tank more effectively.
What Is Nitrate in an Aquarium?
Nitrate is the end product of the nitrogen cycle in your tank. Fish waste, urine and uneaten food release ammonia. Beneficial bacteria convert the ammonia to nitrite, and then to nitrate. In nature, nitrate levels stay very low because of the huge volume of water and the natural water cycle. In a closed tank, nitrate builds up unless you remove it.
New aquarists often confuse nitrite and nitrate. Nitrite is far more dangerous, and any reading in an established tank is a warning sign. Nitrate is less toxic, but it is not harmless, and it is much easier to remove.
The Nitrogen Cycle: The Ecosystem Inside Your Aquarium
The nitrogen cycle is the natural process that turns toxic waste into less harmful compounds with the help of beneficial bacteria. A properly established cycle is essential before you add fish to a new aquarium, and it prevents what is often called “new tank syndrome”.
| Stage | What Happens | Danger to Fish |
|---|---|---|
| 1. Ammonia | Fish waste and decaying matter release ammonia | High. Toxic |
| 2. Nitrite | Bacteria such as Nitrosomonas convert ammonia to nitrite | High. Also toxic |
| 3. Nitrate | Other bacteria (such as Nitrobacter and Nitrospira) convert nitrite to nitrate | Lower, but harmful over time at high levels |
| 4. Removal | Water changes, plants and some algae remove nitrate | This is where you come in |
The Dangers of High Nitrate Levels for Fish and Plants
Nitrate is less dangerous than ammonia or nitrite, and freshwater fish are generally more tolerant than saltwater fish. But long exposure, even at moderate levels, can hurt your fish. According to PetMD’s guide to nitrate poisoning in fish, most fish tolerate nitrate up to about 20 mg/L (the same as 20 ppm), but sensitivity varies by species. Signs can include lethargy, breathing problems and, in severe cases, sudden death.
Chronic high nitrate can also contribute to faded colours, poor appetite and a weaker immune system. It also feeds algae, so an uncontrolled nitrate level often goes hand in hand with an algae bloom. Importantly, there are no visible signs of high nitrate in the water itself, so you have to test.
A common myth: you may see claims online that fish can tolerate hundreds of ppm of nitrate. Some hardy fish survive high levels for a time, but this does not mean those levels are safe. Eggs, fry, discus, shrimp, corals and many saltwater animals need far lower nitrate.
What Are the Optimum Nitrate Levels in an Aquarium?
Sources differ slightly, so treat these figures as a guide. Most aim for under 20 ppm, and some hobbyists tolerate up to 40 ppm, as noted in a vet-reviewed guide to lowering nitrates.
| Nitrate Level | What It Means | What to Do |
|---|---|---|
| 0 to 10 ppm | Ideal, and best for sensitive fish, fry, shrimp and reef tanks | Keep your routine |
| 10 to 20 ppm | Good for most community fish | Test weekly |
| 20 to 40 ppm | Tolerated by hardy fish for a while, but not ideal | Increase water changes, cut feeding |
| 40 to 80 ppm | High. Stress, algae and illness risk rise | Start a series of partial water changes |
| Over 80 ppm | Dangerous for many species | Act now, but lower it gradually, not in one huge change |
Live plants use nitrate as a nutrient, so heavily planted tanks can tolerate and sometimes want a little nitrate. Fish-only tanks, or tanks you are trying to keep free of algae, do best at the lower end.
How to Monitor Nitrate Levels in Your Fish Tank
The only way to know your nitrate level is to test. A liquid test kit compares the colour of your sample with a chart. PetMD notes that vets prefer liquid kits, and that test strips are not considered reliable for diagnosing water quality problems. Test your tank weekly, and also test your tap water, because it can contain nitrate before it ever reaches the aquarium.
This video shows how to measure nitrate in a fish tank:
How to Lower Nitrates in a Fish Tank: 7 Best Ways
You cannot remove every trace of nitrate, but you can keep it under control. For most tanks, aim for 20 ppm or less, and closer to 10 ppm if you keep sensitive species. Overfeeding leaves uneaten food and produces more waste, which means more ammonia and then more nitrate, so cutting back on food is the first fix.
Prevent Nitrate Build-Up First
✅ Feed small amounts that your fish finish in a couple of minutes (see our guide on the right food for your fish)
✅ Do not overstock. If you want more fish, get a bigger tank
✅ Remove uneaten food, dead leaves and other decaying matter
✅ Rinse filter media gently in old tank water, and never replace it all at once
✅ Do not switch the filter off at night: read why you should not turn off the filter at night
1. Change the aquarium water regularly
Regular partial water changes are the most dependable way to keep nitrate down, and PetMD names them as the treatment for high nitrate. Many keepers change around 20 to 30% a week, and heavily stocked tanks may need more. Use a gravel vacuum or siphon to lift waste out of the substrate as you go. Always treat new water for chlorine, match its temperature to the tank, and test it first. If your tap water is high in nitrate, water changes will add it back, so see method 4 below.
If your nitrate is very high, do a series of smaller water changes over several days, rather than one huge change, because a sudden drop can shock fish. See our guides on making tap water safe for fish and choosing the best water for an aquarium.
2. Add live plants
High nitrate in an old, neglected tank is sometimes called “old tank syndrome”. Live plants help because they use nitrate as food. Fast-growing species such as water sprite, water wisteria and hornwort take up the most, and plants also compete with algae. Trim the plants and remove dead leaves, or the nitrate goes back into the water. In saltwater tanks, the equivalent is fast-growing macroalgae (see method 6).
3. Use nitrate-reducing media
Beyond ordinary mechanical and biological media, there are products made to reduce nitrate. Some give bacteria that live without oxygen a place to settle, so they can convert nitrate into nitrogen gas (a process called denitrification). Others bind nitrate chemically. Results vary and some can cause rapid shifts in water parameters, so use them as a helper and not a substitute for water changes. Check the product label for the right use.
4. Use nitrate-free source water
If your tap water already contains nitrate, regular water changes cannot bring your tank below that level. In that case, you can treat your source water with reverse osmosis (RO), deionisation or distillation, and use that water for changes. These processes also strip minerals, so you need to add minerals back (remineralise) before using the water in a freshwater tank, as the vet-reviewed guide above also warns. Test your tap water first to see whether you need this at all.
5. Algae turf scrubber (ATS)
An algae turf scrubber is a device with a lit screen where algae grow and absorb nitrate. You then remove the algae regularly, which takes the nitrate out of the system. It is a biological method used mostly by advanced keepers, in saltwater and sometimes freshwater tanks, and it needs more setup and maintenance than most beginners want.
6. Use a refugium (saltwater)
A refugium is a separate small tank or compartment, often in the sump, where fast-growing macroalgae such as Chaetomorpha (“chaeto”) grow and take up nitrate. Harvesting some of the algae regularly removes the nitrate from the system. Refugiums also provide a safe space for beneficial tiny animals. It is mostly used in saltwater tanks.
7. Deep sand bed (advanced)
A deep sand bed is a layer of fine sand around four inches deep. In the lower layers, where there is little oxygen, bacteria can convert nitrate to nitrogen gas. It can be effective, but it needs careful planning, the right grain size and little disturbance, and it is mostly used in saltwater tanks. If it is disturbed or set up badly, it can cause problems, so I would only suggest it for experienced keepers.
Is 40 ppm nitrate too high?
It depends on your fish. Hardy fish may cope with up to about 40 ppm for a while, but it is higher than most vets recommend, and it can harm fish over time. Many aim for 20 ppm or below, and under 10 ppm for sensitive species.
Nitrite is different: in an established tank, any nitrite reading is a warning to test again and act.
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A Special Care Guide for a New Tank
New tank syndrome refers to the unsafe ammonia and nitrite spikes you can see in a new aquarium. During the initial cycling, as the bacteria colonies settle, ammonia, nitrite and then nitrate rise and fall. Nitrate often climbs significantly in this phase. If it goes very high (over about 80 to 100 ppm), do partial water changes to bring it into a safer range for any livestock.
Fish-in cycling is generally discouraged, because it exposes fish to ammonia and nitrite while the cycle is building. Test regularly, and once ammonia and nitrite stay at zero, your focus can move to keeping nitrate stable with regular maintenance.
Are There Products That Reduce Nitrates?
Yes, but treat them as helpers. Products include nitrate-removing resins and filter pads, denitrifying media, and organic-waste removers that take out waste before it breaks down into nitrate. Some water additives claim to reduce nitrate or algae, but results vary. Plants such as water wisteria are also a natural nitrate remover, but they are not a chemical product.
Whatever you try, read the label, change one thing at a time, and keep testing. No product replaces regular water changes and good feeding habits.
Common mistakes to avoid
- Replacing all the filter media at once. This removes the beneficial bacteria. Cleaning the filter too vigorously can do the same.
- One huge water change to fix very high nitrate. The sudden change can shock fish.
- Not testing your tap water.
- Relying on additives instead of water changes.
- Switching the filter off overnight.
Why Does the Nitrate Level Rise? Common Causes
Several things can push nitrate up:
- Overfeeding and leftover food
- Overstocking, which produces more waste than the tank can handle
- Dirty filters and unvacuumed gravel (detritus)
- Decaying plant material
- Tap water that already contains nitrate
- Too few water changes
Overfeeding and overstocking are two of the commonest causes. If you love keeping more fish, go for a bigger tank rather than overcrowding. Excess light also feeds algae, so see our guide on how much light fish need. If your fish also look unwell, for example swimming oddly, read why a fish swims upside down or sideways.
Frequently Asked Questions
What happens if nitrate levels are too high in a fish tank?
What causes high nitrate in a fish tank?
Are nitrates bad for fish?
How often should I change water to lower nitrates?
How can I lower nitrates quickly?
Can tap water cause high nitrates?
The Parting Thoughts
High nitrate can be a worry, but it is one of the easiest water problems to fix. Test regularly, feed lightly, change water on a schedule, and add live plants. Keeping nitrate at a sensible level protects your fish, helps stop algae and keeps your tank clear.
Key Takeaways
- Nitrate is the end product of the nitrogen cycle. It is less toxic than ammonia and nitrite but harmful at high levels.
- Aim for 20 ppm or less for most fish, and under 10 ppm for sensitive species, fry and reef tanks.
- Regular partial water changes (often 20 to 30% weekly) are the most reliable fix, along with gravel vacuuming and light feeding.
- Live plants, nitrate-reducing media and nitrate-free source water help, but do not replace maintenance.
- Test your tank and tap water, never replace all your filter media at once, and lower very high nitrate gradually.
Did you find this helpful?
What is your tank’s nitrate level, and what works for you? Tell us in the comments, and share this post with another fish keeper.
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