I have never known any Reef salt to contain a significant amount of nitrates. They would have to intentionally add it. You can make a batch of fresh saltwater and test to be sure. 20PPM isn't all that bad, shouldn't have any ill effects on anything. With your recent tank setup it isn't all that unusual of a level. Keep up with good husbandry, lighten the feedings a bit. Add some cheato if you don't already have it in your sump with good lighting and harvest it biweekly and it should come down.
If you want some more advice specific to your setup, post up details about your tank size, water change schedule, equipment and feeding routine