Originally Posted by Byron
There is no magic number for nitrates; the goal is to keep nitrates low, as low as possible, and as close to zero as possible. Nitrate is a toxic substance to all fish. Some species tolerate higher levels than others, but long-term nitrate is harmful. None of the fish we maintain in aquaria live in natural habitats with nitrate levels much above zero, which should tell us something.
You haven't given us the nitrate number for your source water (presumably tap water), and if this is the issue or part of it, there are ways to handle it. If the nitrate is partially or solely occurring within the aquaria, then increasing water changes [50% weekly], lessening the fish density, curtailing feeding and having live plants that are relatively fast-growing (stem and floating plants are ideal) are all ways of handling it. I realize that if these are store tanks, you are under situations that would not be encountered by home aquarists, but the same practices apply.
I believe 20 to 40 ppm Nitrate reading while not being perhap's a magic number,, will be acceptable for the health of most fishes we keep.
As for nitrate being toxic to all fish, studies do show that perhap's some species are much more tolerant than other's, but the studies done were in my view too brief to render a clear picture of long term exposure and effect's on embryo development, fry number's, etc, except for the few game fish that most studies were performed on.
Some studies on tropical's were also performed but they too were perhap's a week or two, and the concentration was increased rather quickly as opposed to observing the long term effect's of exposure.
I would submit that fish don't need to start dying,before they feel the effect's of organic waste in their enviornment. Fish don't get sick ,die,or heal ,overnight.
Nitrate level's as a result of inorganic mineral salt's such as KNO3 that many add to planted aquariums is much less harmful than the break down of organic substances(fish waste ,foods,) and subsequent nitrate reading's from same.
Many other 's in planted tank realm suggest that unless nitrate test is calibrated, (google calibrating nitrate test) that the number's our test kit's produce are questionable to useless.
My own expierience as well as many other's expieriences with nitrate from inorganic mineral KNO3 and
mineral's used in planted aquarium's have shown no ill effect's over a two year period with level's that would not be acceptable to me if as mentioned they were a result of break down of fish poop,fish food's, poor maint,etc.
I am on board with keeping enviornment clean by performing weekly water changes of 35 to 50 percent which will keep the fishe healthy and happy, planted tank, or unplanted.Maybe can get by with fewer water changes in planted aquariums but that is for another topic,.
Is no downside to performing water changes for fishes in closed system in my view.