You misunderstood what I wrote. I will try one more time.
Where does nitrate in an aquarium originate? There are basically only 3 sources:
1. It is the end product of the cycle in tanks.
2. It can be in plant fertilizers.
3. It can be in tap water.
How much nitrate might accumulate in any aquarium depends on the combination of the above three factors. Therefore, to assess a potential nitrate problem requires that one can identify the sources of the nitrate.
You can not keep live plants in a tank without insuring the plants get the proper nutrients. You can add fertilizers and trace elements etc. in any amount or ratio you want as long as you know what the levels are.
Finally, of all the test kits commonly used by hobbyists, the nitrate one is the least reliable/accurate. Therefore, it is not a great idea to blindly trust the results one gets in terms of the absolute values. This leaves one with two ways to attempt to determine if they have nitrate issues or not. One is to pay up to have nitrate levels tested in a more accurate fashion. The other is to "test" the fish. For example, if all other things are relatively fine (i.e not showing obvious levels that are either too high ot too low) so that nitrate is isolated as a likely issue, then one can observe growth rates and other behaviors which would indicate there actually is a nitrate problem. In the more extreme cases it will be obvious as the symptoms or nitrate poisoning are pretty much the same as they are for nitrite. Fish suffocate even in well oxygenated water. And gasping for breath is easy to spot.
But keeping discus in a planted tank also has its own special requirements. They need a bunch of open space. because they are grazer which often feed off the bottom, a fair amount of unplanted space is needed. They can not pick food out of densely planted areas. And then there is the difference in how one keeps young discus vs larger older ones. There is a good article on planted discus tanks here:
http
/www.tropica.com/en/tropica-abc/the-right-fish/discus-aquaria.aspx
But I will assume that you are correct about your nitrate levels, so the question is what to do about them? I would be looking to a nitrate reactor etc. as the least desirable solution. For one it only deals with nitrate and there a lot of other things in discus water one also needs to control. Even if one lowers nitrates, there are still other things that must be removed from the water that are potentially harmful and then there are those things that get used up that must be replaced. So water changes will still be in order. Moreover, if youresort to chemical means for nitrate removal, you need to know if they remove other things one may not want removed and/or if they put back anything one may not want.
I would suggest that an ro/di unit is a better long term solution which is also likely to be less expensive and less effort than a nitrate reactor or chemical based nitrate removal. I would first be looking to use it to cut down the levels of everything in your tap. A 50/50 mix would reduce the nitrate in your tap by 50%. It would also soften your water and make it easier to lower the pH and hold it there if you also need to do this. Next, I would come up with a fertilizing routine that uses less nitrogen (as Nitrate). This means more custom type mixing.
My last observation is that there is a difference between a display tank with adult discus and other fish and plants vs a tank where discus may be spawned or one where there are young small discus. There is a difference between a high tech planted tank and its needs vs the needs of smaller discus being grown out. There are aspects of each that are not good for the other. Therefore, I would suggest you might want to have different setups for a mature discus community vs a grow out tank for discus. In general the simplest soluition is usually the best. What I see as your biggest problem is you are trying to combine two things (small discus and high tech plants) in one tank. This makes things much more complicated and thus much easier to have go wrong.
