The 20ppm isnt optimal of course but is not as horrible as people may think. I dont see that you listed any coral so the 20ppm is almost acceptable. the goal of course is to have zero but in a fish only system it is common.
I would do several larger water changes and see where you are at from there. Once you get them lower they will be easier to stay on top of. I would even say to do a half tank change if you can, I tend to do 50% water changes on most of my tanks with no ill effects. Just make sure salinity and temp match and even check the ph so you dont shock your fish.
From there I would limit my feedings...Do you use frozen food? If so do you rinse the food before adding it to the tank? Might be a good idea to cut back on the nitrates. In addition, if you have any foam blocks or anything be sure to wash them when you change the water in old tank water, nitrates can linger and never seem to go down if thats not done.
Hope that helps a bit.
In fought never ending nitrates in my biocube only to realize that they were never leaving the tank and were in the filter media bags I keepin the tank. lol