High Nitrates in Your Aquarium: Where They Come From and How to Lower Them
Published by Planète Pets · managing editor: Clément Lacaille · how our guides are written
Unlike nitrites, which turn toxic within hours even at low concentrations, nitrates build up slowly and are often overlooked, even though a persistently high level stresses fish and fuels algae growth. A landmark review by Camargo, Alonso and Salamanca, published in 2005 in the journal Chemosphere, detailed the toxic effects of nitrates on aquatic organisms and showed that chronic exposure, even at concentrations long considered safe, affects the growth and reproduction of many species (see the study). Find our other pointers in the care and grooming section.
Contents · 5 sections
Where do nitrates in an aquarium come from?
Nitrates are the end product of the nitrogen cycle: nitrifying bacteria convert toxic ammonia into nitrites, then into nitrates, a far less dangerous form that nonetheless does not disappear on its own without intervention. Overfeeding, an overstocked tank relative to its volume, or infrequent water changes all speed up their accumulation.
What nitrate level should you aim for?
Below roughly 20 to 40 mg/L depending on the species, most freshwater fish tolerate nitrates well; sensitive species and marine tanks call for stricter thresholds. Regular testing with test strips or liquid kits lets you track the trend before it becomes a concern.
How do you bring a high level down?
More frequent partial water changes remain the most reliable and fastest fix, backed up by trimming the feeding ration and regularly cleaning the substrate, where organic waste tends to accumulate. Fast-growing plants also take up part of the available nitrates, an added benefit of a well-planted tank.
Should a one-off spike be a concern?
An isolated spike after overfeeding or adding new fish usually resolves with one or two targeted water changes. It's a level that stays high for several weeks that deserves attention and should prompt a review of the overall maintenance routine.
- End product of the nitrogen cycle, less toxic than ammonia or nitrites but not harmless.
- 20 to 40 mg/L: generally tolerated threshold in freshwater, to adjust by species.
- Regular water changes: the most reliable way to bring a high level down.
- Fast-growing plants as a natural extra layer of nitrate uptake.
Frequently asked questions
Can nitrates kill a fish quickly?
Rarely acutely; the real danger comes from chronic exposure, which weakens the immune system over time.
Does a marine tank tolerate the same levels as a freshwater one?
No, marine species and corals are generally more sensitive and call for stricter nitrate thresholds.
Do nitrates really fuel algae growth?
Yes, along with phosphate, they're among the main nutrients that drive algae outbreaks.
An accessory from our shop for this use: Hang-on isolation and breeding box for aquariums (€28.90) · the whole shop
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.
