Removing nitrate from one's tap is not a simple easy matter. It can be done several ways, but all have a financial and/or space cost.
There is nitrate removing media you can add to a filter.
You can plant the tank. Plants which root in the substrate will cause major changes in what goes on in one's substrate especially in trms of denitrification.
You can set up a separate veggie filter- a tank filled with nitrate loving plants. It can either be used to pretreat changing water or like a sump on the tank itself.
You can take steps to promote denitrifying bacteria either in your filter or in a special filter dedicated for this purpose.
Denitrifying bacteria normally live in the very same biofilm that hosts the nitrifying bacteria. These are actually a type of anaerobic bacteria called facultative. This means they can switch what they do and can so they also can live aerobically. What happens in the biofilm as water penetrates it and moves deeper into it, a host of aerobic bacteria use the oxygen. When there are enough oxygen users built up what happens is they manage to use all of the free oxygen in the water and what they leave in its place is nitrate. So now the facultative anaerobes find themselves in what has become anaerobic part of the biofilm. When there is no free oxygen, they will then switch to using the Nitrate as an electron donor. And the result of all this is the nitrate is broken apart and the nitrogen is released. It then exits the water as the harmless gas that makes up much of our atmosphere.
So every tank has some level of denitrification happening. But usually this natural amount denitrification is not enough to keep up with the amount of nitrate coming in via ones tap and being produced in one's tank. It is possible to use filter and media types designed to foster the growth or more denitrifiers. But this involves larger amounts of media than most normally use and a slower flow rate through it. Most of the filters we use have too high a flow rate and lack the proper media porosity to optimize the colonization of denitrifiers. Canisters are better for this than other filter types. Sumps can be even better. Cleaning ones media of the brown goop also discourages them. Most filters do not have either enough media nor the proper porosity. They still do an excellent job of keeping our tanks clean when we keep up with our maintenance. We need to keep things flowing and the less media one has, the faster it can clog. Another way to deal with all this is to use a Hamburg Matten Filter. These are huge foam blocks through which water passes fairly slowly.
Finally, depending on the other water parameters involved and the needs of a given tank, one can reduce nitrate and other things one needs to reduce in their water by micing one's tap with ro or ro/di water. but this will dilute everything, not just the nitrate.
For most of us fw keepers, nitrate is not normally a problem as long as we do our water changes. However, when nitrate is in one's tap this can become an issue. The other problem is the fact that hobby grade nitrate test kits are pretty poor. They tend to be least accurate in the 0- 20 ppm range. But they can give different results based on how well one does all the shaking of the reagents and the test tubes. Two keepers can use the same brand kit to test the same water and get two different readings.
Depending on how large a tank you have will determine the most effective way to deal with too much nitrate. The nitrate removing media used to remove it is not too costly as long as the amount of water involved is not great.
Here is another interesting method for dealing with nitrate (and phosphate) but which is not cheap but is "natural"-
http
/www.drtimsaquatics.com/control-nitrate-phosphate
