I’ve always heard that changing the nitrate level of a tank by more than 50 ppm within a 24 hour period can shock the fish and cause additional stress so you should decrease it more gradually if the nitrate is super high (it isn’t in this case). Is it your experience that this is a myth? Even if a substance is toxic if your body is used to having it around it can cause even more problems to completely withdraw it suddenly, and nitrate is not nearly as toxic as ammonia and nitrite (or carbon monoxide ?)
I will go into a bit more detail now, to respond. But on the nitrate slowly changing, yes, that is a myth and counters the scientific evidence. I went into this issue with Neale Monks a few years back when it came up here in another thread and I wanted to verify what I had learned from my research before possibly leading someone astray. Getting nitrate out of the aquarium water as rapidly and completely as possible will always benefit the fish and may prevent further problems.
A couple of additional points now. Obviously, we are working with the water parameters being reasonably close between tank and source (fresh) water coming in. Parameters being the GH, pH and temperature as I'm sure you know, and only if these are significantly out could large water changes harm the fish. Changing as much of the water regularly as feasible is intended to have the effect of more stability of the chemical and biological system, and that is key to healthy fish. So any one of the nitrogen forms that are encountered in the aquarium (ammonia/ammonium, nitrite, nitrate, nitrogen gas) should always be removed as quickly and effectively as possible--and preferably through a permanent preventive process like live plants that are fast growers , not overstocking, not overfeeding, regular substantial water changes, etc. Then we also have natural denitrification that occurs within the substrate thanks to the species of heterotrophic bacteria that utilize nitrate by consuming the oxygen within nitrate and releasing nitrogen gas. They do not require free oxygen in the water so they are facultative anaerobes, and generally occur in what we term “dead spots,” which occur when water movement is stopped and thus no oxygen is available. These are the good guys among heterotrophs, since de-nitrification is important in a healthy aquarium; the nitrogen gas dissipates out of the water into the air. Nitrate occurring in the source water itself is an added issue but one that must be factored in and dealt with, depending upon the level.
Fish do not get used to nitrate, it actually is continually weakening them, and I'd like to expand on this with study evidence included. Most scientific nitrate studies have been in waste water and agriculture, not the fish aquarium hobby, and thus not really looking into specific aquarium issues so they tend to get overlooked and ignored by most hobbyists [more below on this]. You mention nitrate at 50ppm and while I agree with the next phrase that keeping it below 20ppm is preferable, I would note that the US Environmental Protection Agency classes nitrate above 44 ppm (using the hobby measurement of NO3 nitrate for an equal comparison) as dangerous to humans; fish living permanently in such water can be expected to have serious issues, so I would consider 50 ppm very worrying. Copper for example at levels safe for humans is toxic to fish, which is why when new copper water piping was used in houses the result were dead tank fish.
High levels of nitrate, above 40 ppm, have been shown to slow fish growth, suppress breeding, and depress the immune system making the fish much more susceptible to disease. While different fish species show some variation in tolerance, a level below 20 ppm is recommended, and preferably below 10 ppm. After all, most of our fish occur in waters with nitrate so low it can scarcely be measured. Fish that have a physiology evolved to not have to deal with nitrate will always be detrimentally affected, the degree depending upon the species, individual fish, and nitrate level, plus the period of exposure obviously. One study ["Nitrate Toxicity to Aquatic Animals: A Review with New Data for Freshwater Invertebrates," [link follows citation] provided this synopsis:
Published data on nitrate (NO3-) toxicity to freshwater and marine animals are reviewed. New data on nitrate toxicity to the freshwater invertebrates Eulimnogammarus toletanus, Echinogammarus echinosetosus and Hydropsyche exocellata are also presented. The main toxic action of nitrate is due to the conversion of oxygen-carrying pigments to forms that are incapable of carrying oxygen. Nitrate toxicity to aquatic animals increases with increasing nitrate concentrations and exposure times. In contrast, nitrate toxicity may decrease with increasing body size, water salinity, and environmental adaptation. Freshwater animals appear to be more sensitive to nitrate than marine animals. A nitrate concentration of 10 mg NO3-N/l (USA federal maximum level for drinking water) can adversely affect, at least during long-term exposures, freshwater invertebrates (E. toletanus, E. echinosetosus, Cheumatopsyche pettiti, Hydropsyche occidentalis), fishes (Oncorhynchus mykiss, Oncorhynchus tshawytscha, Salmo clarki), and amphibians (Pseudacris triseriata, Rana pipiens, Rana temporaria, Bufo bufo). Safe levels below this nitrate concentration are recommended to protect sensitive freshwater animals from nitrate pollution. Furthermore, a maximum level of 2 mg NO3-N/l would be appropriate for protecting the most sensitive freshwater species. In the case of marine animals, a maximum level of 20 mg NO3-N/l may in general be acceptable. However, early developmental stages of some marine invertebrates, that are well adapted to low nitrate concentrations, may be so susceptible to nitrate as sensitive freshwater invertebrates.
Neale told me that small species, and also cichlids, are especially sensitive to nitrates; a level of 20 ppm will cause many of the cichlid problems, and there is now some evidence that Malawi Bloat and other diseases may be more the result of nitrate levels than other issues; this is certainly becoming common thinking on the cichlid site. I don't think there is much doubt that nitrate is detrimental to all freshwater species, and keeping them as low (near to zero) as we can is going to benefit the fish we keep. I know you agree with this principle, but getting to this stage is best done rapidly, given the risks along the way.
Nitrate is showing as 9.68 & water hardness 7.63
