5.4.1. Nitrate in Depth

  Nitrates, Nitrogen

All discussions of research papers on nitrate must be predicated with a caution. The 10 ppm of nitrate of the scientists, (the Hach test kit and the Lamotte test kit) is a 44 ppm test level of nitrate via the “normal” aquarium test kits (API aquarium test kit, the Seachem Multitest kit, the Sera Nitrate test, the Salifert test kit and the API test strips). So most of the scientific studies need the limits to be multiplied by 4.4 to come up with proper “nitrate” levels for the hobbyist.This is because scientists look at ONLY the nitrogen in the nitrate, they do not look at the oxygen in the nitrate (nitrate is one nitrogen atom and three oxygen atoms). The API and other aquarium tests look at the entire nitrate; nitrogen AND oxygen.


Those who say that tropical fish have poor health if kept in water over 20 ppm of nitrate are being far too cautious. There is no evidence that anything lower than 80 ppm nitrate per the API test even AFFECTS fish. There is evidence levels down to 40 ppm can AFFECT shrimp and axotyl. Note that the obvious problem is that AFFECT and HARM are not synonyms.

Nitrate is not very toxic to higher animals which have well developed livers, like fish. The reason is simple. The liver is an amazing chemical factory. In the liver nitrate is converted to ammonium (NH4). This ammonium in humans is converted to urea and excreted through the kidneys. In fish the ammonium is excreted directly out the gills as ammonia gas. Thus adult fish and adult humans handle nitrate quite well and it has a low toxicity for both.

Research Papers

The first number to consider when looking at nitrates and fish is the lethal toxicity. This is something along the lines of 72-h lc50 mg 1−1 −N. This is the concentration (expressed in parts per million of nitrate NITROGEN ONLY) where 50% of the fish died in 72 hours. This number always must be multiplied by 4.4 to get the number the API test kit gives.

If one puts all the papers together and looks at the LC50 per the API test hobbyists use,  one gets the following chart:

It is apparent that the first five fish, which are the only aquarium fish in the list, have lower numbers than the other fish. But the average is still 979 ppm nitrate, even for aquarium fish. This is VERY high. And obviously the other data is HUGE. Nitrate is simply not very toxic to fish.

Each Paper

Per one paper: “Recirculating Aquaculture Tank Production Systems”, Masser et. al. Southern Regional Aquaculture Center, March 1992:

Nitrate, the end product of nitrification, is relatively nontoxic except at very high concentrations (over 300 ppm)”.

Considering that this article uses nitrate nitrogen, which means the aquarium owners API test kit has to hit 300 x 4.4. or  1,320 ppm nitrate in order for nitrate to be toxic per this authority.

Milligrams per liter is the same as parts per million (ppm). This is because a milligram is one one thousandths of a gram and there are one thousand grams in a liter.

To convert the nitrate nitrogen to what fishkeepers use as nitrate (total nitrate, i.e. both the nitrogen and the oxygen in NO3) one must multiply the researchers’ number by 4.4. Thus at 72 hours 50% of the guppy fry were dead when presented with 875 ppm of nitrate. That is a very low toxicity, especially considering this was for fry.

There is one paper which pretty much sums ups the effect of nitrate on fish (“Histopathological Changes in the Liver of a Farmed Cyprinid Fish, Cyprinus carpio, Following Exposure to Nitrate”,  Iqbal et. al, 2005):

“LC50 of nitrate to Cyprinus carpio, according to Reed-Muench method, were 995 ppm for 48-hr and 865 ppm for 96-hr.”

So the levels of toxicity and death for these carp in this test were 865 x 4.4 or 3,806 ppm of nitrate with the API test kit.







5.4.1. Nitrate in Depth