Given that fish are sensitive to prolonged exposure to high nitrates, there is a reasonable level present in the water and fish water is being converted too then the only outcome I can see is ever increasing levels of nitrates? (Even with 50% weekly water changes)
This is another convoluted misconception perpetuated primarily by Nitrate test kit vendors. They want you to be afraid of Nitrates so that you will buy more of their silly kits. Fish breeders and Discus fanatics took up the baton years ago and have run with it since. So now everyone is inoculated with this idea without ever researching it for themselves.
While it is true that almost any agent in the water can be toxic, toxicity is always a function of concentration. Exactly what concentration induces toxicity is the key.
A lot of the data for Nitrate toxicity comes from studies performed by Environmental Protection Agencies on temperate freshwater waterways, which have little to do with animals native to tropical waterways. Tropicals are much less affected by Nitrate levels than are temperate species.
Even so, if we look at some of the data as an "indication" we will see, as Edvert mentions, that NO3 has a benign effect compared to the effects of NH3 and NO2. It's often difficult to separate the effects of these three Nitrogen compounds because they coexist in the water. Nitrate accumulates in the water as an end product of NH3 and NO2 conversion. So we "measure" NO3 accumulation and assume that the damage is caused by this accumulation, but this is an illusion. The damage is caused by NH3 and NO2 as they are being produced.
Just for grins, have a look at some studies of NO3 toxicity. In these studies, the researchers focus on the concentration of the Nitrogen part of the Nitrate molecules. In NO3 "N" is only 23% by weight of the molecule, so you'll see the expression "NO3-N" expressed in values of mg/L (ppm). So, to achieve a concentration of 1ppm of NO3-N it requires 4.4ppm of NO3. This is another area of confusion where folks completely misunderstand the numbers being reported and they assume that a number like 10ppm NO3-N is the same as 10ppm NO3, when it actually means 44ppm NO3.
Also keep in mind that there is the issue of "acute" toxicity, where the fish are exposed to the compound at a high enough concentration to kill them outright (or within a short period of time), versus "chronic" exposure, where physiological effects of exposure are evaluated over the long term. In the case of acute exposure, you will see an expression such as "72 LC50", which can be interpreted as; "the Lethal Concentration required to kill 50% of the subjects within 72 hours".
Have a look at
https://www.sciencedirect.com/science/article/pii/S0045653516314436
If you scroll down to paragraph 3 you can see the summary results for juvenile Rainbow Trout:
Acute toxicity testing with juvenile rainbow trout resulted in median lethal concentrations (
i.e., 96-hr LC50) that ranged from 808 mg/L NO3-N in very soft water to 1913 mg/L NO3-N in hard water. Testing with 6–8 day old
H. azteca produced 96-hr LC50 values that increased from 168 to 485 and 921 mg/L NO3-N in soft, moderately-hard and very hard water, respectively (
Table 3). A clear reduction in toxicity with increase in water hardness was apparent in tests using both species (
Fig. 1).
So on average, to kill 50% of the juveniles within 4 days they needed between 3555ppm NO3 and 8400ppm NO3 depending on the water hardness.
Even for the invertebrate fry the numbers are 740ppm to 4050ppm NO3 concentration.
These numbers are outrageous and we never get anywhere close to them.
For chronic exposure studies on the same species have a look at
https://ac.els-cdn.com/S01448609140...t=1519747515_0d3497a7c95083031e173032640cfac7
According to that study:
Although most water quality variables were controlled, significant differences between treatments for the concentrations of other water quality parameters inhibited definitive conclusions regarding the effect of NO3-N. However,due to the unlikely toxicity of confounding water quality parameters, study results provided strong evidence that relatively low NO3-N levels, 80–100 mg/L, were related to chronic health and welfare impacts to juvenile rainbow trout under the described conditions.
Again, these numbers are about 350ppm to 440ppm NO3, far higher than what we ever see, and even these numbers are suspect due to anomalies with the ability to maintain consistent water parameters.
Here is a interesting article more relevant to tropicals, which studies the effects of NO3 concentration on pathogens attacking guppies
https://www.sciencedirect.com/science/article/pii/S0160412016300885
While the authors quite rightly condemn agricultural eutropification of natural water systems, they discovered an interesting anomaly:
Excessive fertilisation is one of the most pernicious forms of global change resulting in eutrophication. It has major implications for disease control and the conservation of biodiversity. Yet, the direct link between nutrient enrichment and disease remains largely unexplored. Here, we present the first experimental evidence that chronic nitrate enrichment decreases severity and induces protection against an infectious disease. Specifically, this study shows that nitrate concentrations ranging between 50 and 250 mg NO3−/l reduce Gyrodactylus turnbulli infection intensity in two populations of Trinidadian guppies Poecilia reticulata, and that the highest nitrate concentration can even clean the parasites from the fish. This added to the fact that host nitrate pre-exposure altered the fish epidermal structure and reduced parasite intensity, suggests that nitrate protected the host against the disease.
In this case the concentration levels were reported in direct NO3 ppm - 50ppm to 250ppm. The test was not to determine the lethality to the fish, but the lethality to the parasite which infected the fish or which was present in the water.
This is an unusual study and I find it fascinating in light of all the hand wringing and hysteria that goes on about NO3. Who knows what other pathogens are inhibited by NO3.
If you search the journals, you can find loads of NO3 toxicity studies and you'll always see number in this range.
Discus breeders are quick to argue that elevated NO3 levels inhibit growth but have a look at a study using highly sensitive salmon:
https://ac.els-cdn.com/S00448486140...t=1519746427_f52496b3e5eddd36d64c5bbd7d2c2d41
Animals were exposed to 5.2, 10.3 or 101.8 mg/L nitrate-N for 27 days. Upon completion of the trial, the animals were euthanized
and bled by puncture of the caudal vein. Mean plasma nitrate/nitrite concentrations increased significantly with increasing ambient nitrate-N concentration. Plasma testosterone concentrations displayed a highly significant non-monotonic dose response to increasing nitrate-N concentration, and were elevated at 10.3 mg/L nitrate-N. Plasma 11-ketotestosterone, total thyroxine and total triiodothyronine concentrations did not differ
significantly between treatments. These results suggest that elevated nitrate can interfere with the synthesis or metabolism of sex steroids, but that Atlantic salmon may be relatively insensitive, in terms of growth and most endocrine endpoints examined, to nitrate-N concentrations up to 101.8 mg/L, and are a promising candidate for production in RAS.
The tested NO3 levels were 21ppm, 45ppm, and 448ppm. If you look at the charts in this article you'll see that there was very little difference in growth rate of the fish.
Do a search of NH3 and NO2 toxicity and you see that the numbers are orders of magnitude lower. These are the real dangers to fish are these two compounds, not to mention that if they are produced organically, their conversion to NO3 requires the consumption of Oxygen, which further is damaging to the fish.
Inorganic NO3 added to the tank does not consume Oxygen and as shown in the studies, the levels we add to the tank has a negligible effect on the fish's health.
You can therefore dose the standard amounts in accordance with EI without any fear whatsoever. What is necessary however is to perform large weekly water changes because the addition of CO2 plus nutrients generates a very high metabolism which produces large amounts of organic waste. It is this waste, which feeds the bacteria an lowers the Oxygen content as the waste breaks down to NH3 and the bacteria feeding on the waste use the Oxygen to convert the NH3 to NO3.
Cheers,