Nitrate, NO3-N, measured in ppm, is the third and last measurement used to determine the "health" of the bio-converter. Nitrate is produced by one of the autotrophic bacterial colonies by combining oxygen and nitrite. This occurs both in the bio-converter and to a lesser degree on the walls of the pond. A zero nitrate reading, combined with a non-zero nitrite reading, indicates the nitrite-nitrate bacterial converter action is not established. Test kits are available with dual droplet or pill form with color charts. The recommended test kit range 0 - 200 ppm. A nitrate test kit is considered nice to have but not required for the average pond. In an established pond with part of the routine maintenance including 5% to 10% water change outs every week or two, nitrate levels will normally stabilize in the 50-100 ppm range. Concentrations from zero to 200 ppm are acceptable but should normally be below 100.
Where ammonia and nitrite were toxic to the fish, nitrate is essentially harmless. There have been reports that high nitrate levels may weaken the colors in Koi, but there have also been reports that high nitrate levels can enhance the colors. Similarly, I have read reports, fortunately not in the same article, that high nitrate levels will both stimulate and suppress spawning activity. If the nitrate concentration gets too high, the nitrite-nitrate converting bacteria may not be able to do their job effectively resulting in a raised nitrite level. Nitrate is the end result of the nitrification cycle and is very important to plants in their life cycle. This is why the plants in your garden can flourish from being watered with the waste water from your pond (assuming you haven't added too much salt).
Note the large difference in the ranges of the test kits being used to measure nitrate (200 ppm) as opposed to those for ammonia and nitrite (1-4 ppm). Assuming our the bio-converter was converting the equivalent of 1 ppm of ammonia to the equivalent of 1 ppm of nitrite to the equivalent of 1 ppm of nitrate per day, it would take 100 days or over three months, (longer with any water change outs), for the nitrate levels to build up to the 100 ppm level. The nitrate concentration is controlled naturally through routine water change outs and to a lesser degree through plant/algae consumption.