If the nitrates are 0 ppm, you know that all the nitrogen is being used. This may indicate that some plants are starving for nitrogen. It also might indicate that a perfect balance has been achieved, but that is unlikely. Unfortunately, potassium is difficult to measure in the water. If there are enough nitrates, there is usually enough potassium.
Some fertilizers contain additional potassium and can be used to be on the safe side.
These are taken in by the plant in ion form. Sulfur, calcium and magnesium are usually found in tap water. This can be remedied by adding calcium and magnesium sulfate in small quantities. Phosphorus can be measured in the water and should be present in quantities less than 0.2 ppm of phosphate. To prevent this, chelated iron mixtures can be used.