You are checking the water before adding in the total amount correct? I have found that it isnt always wise
to mutiply and add... it just wasnt close enough to what I was shooting for. I always try to slowly adjust to avoid back and forth, plus give the water time mix for a more accurate reading. I could be wrong in all this but I have made a habbit of learning from my mistakes and you could very well know what the heck your doing..lol
I cant answer your question on the Nitrogen though, sorry.

Please state the concentration of Nitric Acid you are using now. I will look into some info. to answer your question as soon as I know the% of Nitric Acid.

I need to know the percentage of Nitric Acid you are using so I can calculate how much Nitrogen or Nitrate you are adding to your tank.
Safwat

The percentage of nitrogen in nitric acid is 22.22%. If you are adding 18 cc of nitric acid to one gallon of water you are adding too much nitrogen to the nutrient solution and it will cause an imbalance of other nutrients.
To have around 100 ppm nitrogen, add 1.7 cc of nitric acid to one gallon of nutrient solution.