I have no option but to disagree with the view that nitrates are not harmful, and allowing them above 20ppm is fine. It is not. And I have the science to back this up.
Nitrates in North American rivers may be high, but many of our rivers are polluted to some extent and the fish are weak or dying in many of them. As for the waters native to most of our aquarium fish, they are zero nitrate or very near zero, and there are many test results available that prove this. The highest level for nitrates in the Amazon itself was 1.24 ppm, and in the Orinoco the level was barely detectable.
Dr. Neale Monks has recently written several times in PFK that nitrates should never be allowed to rise about 20ppm for any fish. He singled out cichlids in one case as most probably weakening with nitrates at 20ppm. He notes that many fish cannot tolerate this level at all.
Nitrate like ammonia and nitrite is a form of nitrogen, and all three are toxic to all fish. We know that ammonia and nitrite are deadly very quickly and at very low levels, and we also know that nitrate is not. But it is still toxic. What is not fully understood yet is just how far an elevated level of nitrate can be continued before the fish will die from it, or if it is the secondary issues caused by the nitrate that are the direct cause of the fish dying.
In one study, Fathead Minnows were killed by a nitrate level of 2.3 to 7.6 ppm within 96 hours, and other species withstood the nitrates longer. One thing this does prove is that nitrates are toxic and do kill, which dispels that myth.
Other tests showed that short-term exposure to less than lethal levels of nitrate did have physiological impacts on the fish, among them:
- Affects antibody production
- Increased number of immature red blood cells
- Lowered level of mature red blood cells (anemia)
- Higher count of monocyte (a specific white blood cell)
- Higher count of neutrophil (a specific white blood cell that is especially destructive to microorganisms)
- Higher count of TLC - Thrombocyte-like cell (a blood cell of nonmammalian vertebrates that promotes blood clotting)
- Higher levels of creatine (A nitrogenous organic acid found in muscle tissue that supplies energy for muscle contraction)
- Higher calcium values in the blood
- Lower Chloride values in the blood
- Autopsy revealed damage to the spleen, liver, and kidneys
Other conclusions were that Nitrate damages the gills and kidneys affecting osmoregulatory ability (ability of the fish to regulate fluid levels and release toxins, something we do via urination, something they do via osmoregulation). The observed changes are the result of a pathological response to nitrates and not of a generalized stress response.
So what does the abnormal blood chemistry indicate? In short, it means the fish are suffering from infection, severe physical stress, and tissue damage. Their blood is incapable of distributing sufficient oxygen, the immune system is in overdrive and has become deficient, and the kidneys are failing.
It is true that these results are only because the Nitrates were severely excessive. So the question is at what point do these impacts begin? Is it possible that these physiological changes can be detected in fish subjected to much lower concentration of nitrates over a longer period of time? Unfortunately, research on the subject is lacking, but there is evidence that sub-lethal nitrate toxicity actually begins at lower concentrations, with the physiological impacts increasing as either the concentration of nitrates increase or the duration of exposure increases.
Studies performed on Gambusia in Florida springs discovered that decreased fertility rates were caused by nitrate in concentrations as low as 1.5ppm.
A compiled review of prior testing conducted in Spain at the Universidad de Alcala suggests that the effects of nitrate toxicity in the most sensitive freshwater species can begin in concentrations as low as 2ppm and that long term exposure to nitrates in concentrations of 10ppm are known to adversely effect rainbow trout, Chinook salmon, and cutthroat trout.
We should not confuse tolerance with good health. This is no different than keeping a 14-inch Oscar in a 20g tank. Yes it is possible, and the fish may even live for its expected lifespan, but at what cost?