Nitrates toxicity is very low for fish/shrimps. Even sensitive fish can cope with 40mg/l NO3 level. I have keept accidently level of 80 mg/l NO3 over 6 weeks and I didn't notice any dead shrimp or fish.
In planted tank (high plant biomas) with good light NO3 is used up quickly by plants, as it is the biggest macronutrient, and it is often required to add this nutrient to the tank. Only carbon is required in bigger quantities than nitrogen (about 30x more carbon is required than nitrogen) and carbon should be called megamacronutrient 😀 .
Easy way to add NO3 is to add KNO3 salt which should be available in many gardening stores.
Edit:
I don't know what kind of ferts you are using if any, but if you've got low NO3 in your tank, it is very likely that you can also have PO4 and K deficiency. Hobby tests quality is another story.