What is nitrate?

Nitrate is the end point of the process of nitrification which is part of the Nitrogen Cycle. Its chemical formula is NO3- and it is formed by the oxidation of nitrite by aerobic bacteria:

Reporting of nitrate and test kits

The reporting of nitrate can be quite confusing as there are multiple methods in use, different test kits use different measures and there is even variance in the scientific literature.

Most hobbyist test kits give readings in either nitrate (NO3-) or in nitrate-N (NO3-N). The former reports the total amount of nitrate present whereas the latter only reports the amount of nitrogen present as nitrate - i.e. excluding the 3 oxygen atoms. For the same concentration of nitrate, a reading reported as nitrate will be 4.4 times higher than one reporting nitrate-N. This is because one nitrogen atom has an atomic mass of around 14 and oxygen has an atomic mass of 16: (14 + 16 + 16 + 16) / 14 = ~4.4.

The next confusing area (and this doesn't only apply to nitrate) is the use of ppm. ppm stands for parts per million and is frequently used for reporting concentrations. The main problem with the use of ppm is it is not clear whether it represents a weight/weight measure (e.g. mg/kg), a weight by volume (e.g. mg/L) or a volume by volume (μL/L). Fortunately, when dealing with seawater the difference between mg/kg and mg/L is not all that great (around 2.5%) and with the accuracy of the test kits, the differences are usually unimportant.

Table 1 shows some available test kits for nitrate, how they report nitrate and the units they use. Also included are the ranges and the claimed number of tests.

Table 1: Reporting methods and units for various nitrate test kits
Brand Reports Units Range Range Nitrate Number of tests
Aquarium Systems (fasTesT) Nitrate-Nitrogen mg/L 0 - 100 mg/L 0 - 440 mg/L
Hagen Nitrate mg/L 0 - 110 mg/L 0 - 110 mg/L 80
LaMotte (AQ-4 & AQ-5) Nitrate-Nitrogen ppm 0.25 - 10.0 ppm 1 - 44 ppm 50
Salifert Nitrate ppm 0 - 100 ppm 0 - 100 ppm 60
Seachem Nitrate mg/L 0 - 50 mg/L 0 - 50 mg/L >75

One other method of reporting nitrate, usually in scientific papers, is the molar scale or molarity. This reports the number of moles of nitrate per litre of water. (A mole is 6.02214199 × 1023 molecules.) As molarity measures the number of molecules rather than their mass, the number of nitrate molecules is the same as the number of nitrogen molecules present as nitaite.

What concentrations of nitrate are found on natural reefs?

Kleypas et al. (1999) report the minimum and maximum nitrate concentrations for coral reefs around the world are 0.00 and 3.34 μmol litre-1 (0.00 and 0.21 mg/L) respectively and that 90% of all reefs have nitrate less than 0.60 μmol litre-1 (0.04 mg/L).

Toxicity of nitrate
Nitrate is generally non-toxic and most organisms can tolerate very high concentrations, over 100 mg/L. Table 2 lists the determined nitrate toxicity for a number of organisms from various taxa. The 96h LC50 indicates the Concentration that is Lethal to 50% of individuals during exposure for 96 hours. The "Safe level" is determined by various methods, including assuming 10% of the 96h LC50.

Table 2: Nitrate toxicity for various organisms under marine conditions
Organism 96h LC50
"Safe level"
  Beaugregory, Stegastes  leucostictus >13,200 Pierce et al. (1993)
  Filefish, Monocanthus hispidus 2,520 Pierce et al. (1993)
  Sea urchin, Paracentrotus lividus 440 Basuyaux and Mathieu (1999)
  Juvenile tiger prawn, Penaeus monodon ~10,000 ~1,000 Tsai and Chen (2002)
  Abalone, Haliotis tuberculata 440-1,100 Basuyaux and Mathieu (1999)
  American oyster, Crassostrea virginica ~16,600 (juv)
~11,450 (adult)
Epifano and Srna (1975)

As can be seen from the values presented, tolerance for nitrate is high in many organisms. It should be noted, however, that different stages of the life cycle of an organism may have different tolerances to nitrate. For example, while juvenile and adult tiger prawns, Penaeus monodon, may have a high tolerance, Muir et al. (1991) found that nitrate concentrations as low as 1 mg/L caused significant mortality in larval P. monodon after exposure for only 40 hours.

While nitrate is not very toxic, it may cause problems for some organisms. Frakes and Hoff (1982) found that the growth rate of juvenile clownfish, Amphiprion ocellaris, was lower in water with nitrate at 440 mg/L compared to 70 mg/L.

Nitrate and corals

Nitrate concentrations as low as 0.06 mg/L can cause problems for symbiotic stony corals. Marubini and Davies (1996) found that calcification rates of Porites porites and Montastrea annularis decreased when the corals were subjected to increasing nitrate concentrations. The calcification rate of P. porites dropped by around 50% when the nitrate concentration was held between 0.5 and 5 μmol litre-1 (0.03 - 0.5 mg/L) for 40 days. For M. annularis, calcification was reduced by 50% for a nitrate concentration between 0.2 and 1 μmol litre-1 (0.01 - 0.06 mg/L) for 30 days. For both corals, further increases in nitrate concentration, up to 20 μmol litre-1 (1.2 mg/L), had little effect on calcification rates.

What concentration of nitrate should be the goal for a marine aquarium?

The goal for any marine aquarium should be zero detectable nitrate, however, depending on the organisms to be kept, nitrate may not be much of a concern.

How do you reduce nitrate?


Basuyaux O. and Mathieu M. 1999. Inorganic nitrogen and its effect on growth of the abalone Haliotis tuberculata Linnaeus and the sea urchin Paracentrotus lividus Lamarck. Aquaculture 174:95–107.

Epifanio C.E. and Srna R.F. 1975. Toxicity of ammonia, nitrite ion, nitrate ion, and orthophosphate to Mercenaria mercenaria and Crassostrea virginica. Mar. Biol. 33:241-246.

Frakes T. and Hoff F.H. 1982. Effect of high nitrate-N on the growth and survival of juvenile and larval anemonefish, Amphiprion ocellaris. Aquaculture 29:155-158.

Kleypas J.A., McManus J.W. and Menez L.A.B. 1999. Environmental limits to coral reef development: where do we draw the line?. Amer. Zool. 39:146-159.

Marubini F. and Davies P.S. 1996. Nitrate increases zooxanthellae population density and reduces skeletogenesis in corals. Mar. Biol. 127:319- 328.

Muir P.R., Sutton D.C. and Owens L. 1991. Nitrate toxicity to Penaeus monodon protozoea. Mar. Biol. 108:67–71.

Pierce R.H., Weeks J.M. and Prappas J.M. 1993. Nitrate toxicity to five species of marine fish. J. World Aquacult. Soc. 24:105–107.

Tsai S. and Chen J. 2002. Acute toxicity of nitrate on Penaeus monodon juveniles at different salinity levels. Aquaculture 213:163-170.

Last updated: June 12, 2005