Nitrates should be as close to 0 as possible. Anything reading between 0 and 5 ppm is ok. However, once you go over 5 ppm, you need to reconsider what you're doing by way of water changes, feeding, filters, etc. I would recommend you do a partial water change, 20%. Also, check your feeding, as you might be overfeeding. 20 ppm is not good, and is 4x the maximum safe level for nitrates.