There have been many recommendations to keep nitrates low (below 25 ppm) in our aquariums. I'm not sure why, because the scientific literature and experts repeatedly
imply that nitrates aren't very toxic. At the end of this letter, I have listed documented values from the scientific literature.
What concerns me is that hobbyists are testing and worrying about something not very significant in terms of fish health. One of my tanks runs for long periods with 40-100 ppm nitrates. Meanwhile, hobbyists may be ignoring major toxins like nitrite and ammonia or incorrectly attributing disease problems to fish weakened (?) by nitrates.
The only downside of high nitrate levels is that sometimes they can lead to nitrites in the tank. Nitrite accumulation can occur due to incomplete nitrification or nitrate respiration (a bacterial process promoted by high nitrate levels and anaerobic conditions). I've described how this can happen in my book, pp 65-66. Nitrite, which is very toxic, should be kept below 0.02 ppm.
HOWEVER, the most direct means of monitoring rare potential problems due to high nitrates is simply to test for nitrite.
Finally, remember that scientific papers almost always use the term NO3-N, which can easily mislead hobbyists into thinking nitrates are more toxic than they really are. One NO3-N is equivalent to 4.4 nitrates. So if a scientific paper says guppys die at 200 ppm NO3-N, it means 880 ppm nitrates for hobbyists. I could be wrong here, but I think that most hobbyist testkits express nitrates as NO3, not NO3-Nitrogen?
Recommendations to keep water nitrates low are not a bad idea. We all like to keep our fish in relatively clean water.
But are nitrates toxic? You be the judge. After I finished compiling this data, I wonder why I even bothered testing for nitrates.
Anyway, here is the data:
Nitrate’s Effect on Fish and Invertebrates.
Largemouth Bass 420 ppm None on mortality or growth over a 5 month test
Channel Catfish 420 ppm None on mortality or growth over a 5 month test
Chinook salmon 5,800 ppm 50% dead in 3 days (96h LC50)
Rainbow trout 6,000 ppm 50% dead in 3 days (96h LC50)
Channel catfish 6,200 ppm 50% dead in 3 days (96h LC50)
Bluegill 1,800-8,800 ppm 50% dead in 3 days (96h LC50)
Guppy 836 ppm 50% dead in 3 days (96h LC50)
American Oysters 11,500 ppm 50% dead in 3 days (96h LC50)
Japanese Oysters 4,400 ppm No effect on growth
Sturgeon, juveniles 4,400 ppm 50% dead in 3 days (96h LC50)
*Note: I converted documented values (expressed as mg/l of NO3-N) to ppm nitrate (NO3). One mg/l or ppm of NO3-N is equivalent to 4.4 ppm of nitrate. [Most hobbyist test kits measure nitrates, not NO3-N.]
Hamlin HJ. 2006. Nitrate toxicity in Siberian sturgeon (Acipenser baeri). Aquaculture 253: 688-693.
Russo RC. 1985. Ammonia, nitrite, and nitrate. In: Rand GM and Petrocelli SM (Eds.), Fundamentals of Aquatic Toxicology. Hemisphere Publishing Corp. (Washington, D.C.), pp. 455-471.
Spotte S. 1979. Fish and Invertebrate Culture. Second Ed. Wiley-Interscience Publications (New York).