You're correct but research has shown that levels of 80+ppm have little long term effects on many freshwater species (I've been searching for weeks for the paper I read as I'd quoted some info from it in another thread but I can't find it online anywhere) obviously some species and juvenile fish especially are more sensitive than others and lower is always better but I wouldn't consider 40ppm high at all. I'd actually doubt the accuracy of the hobby test kits, nitrate tests especially are woefully inaccurate and not precise at all given they're mostly dependent on x number of drops being used.
I remember that thread, and would still be interested in seeing the study mentioned. I cannot agree that nitrate does not matter, as this goes against all known scientific evidence and reason.
Most of the nitrate studies deal with non-aquarium fish, that is true. And most are also not long-term but more designed to find out at what level nitrate is likely to kill the fish outright. One cannot apply such results to all fish.
I went into this issue in detail with Neale Monks. Neale has consistently advised keeping nitrate below 20 ppm for all fish, and beyond that as low as possible. Some fish species are more susceptible to nitrate short-term than other species; Neale specifically mentioned all cichlids and mollies. Over on the cichlid forum they advise 20 ppm as the maximum for nitrate, citing nitrate as probably linked to Malawi bloat and hole in the head. Neale said this is certainly a risk, and he suggested it is best to consider nitrates as simply more like stress that is weakening the immune system and wearing down the fish. And this of course makes it more susceptible to diseases it would otherwise be able to fight off.
Given that none of our aquarium fish live in water containing nitrate at levels anywhere near what we are considering, and their physiology has evolved over thousands of years to function best in such environments, it is safe to assume the fish will be healthier with nitrate as low as possible. And healthy fish is hopefully what we all want.
Live plants have been suggested, and it is true that these will help to keep nitrates low. But this is not going to matter much in most setups. Plants take up ammonia/ammonium as their preferred source of nitrogen, and except in a high-tech planted tank using diffused CO2, very bright lighting to drive photosynthesis, and daily nutrient supplementation, their uptake of nitrate will be minimal. Plants only turn to nitrate when ammonia/ammonium is no longer sufficient; the plants have to internally change the nitrate back into ammonium, and this uses valuable energy so it is only going to be significant when all the other factors are present. Nitrate will be low, usually less than 5-10 ppm, often at zero, in natural or low-tech planted tanks that are not overstocked but biologically in balance; this is because there is so little nitrate occurring because the plants grab most of the ammonia/ammonium and little gets through the nitrifying bacteria process, and what does may get snapped up by some of the plants, all depending upon the species, number, and fish load.
Byron.
