The interesting thing is you don't seem to have any hair or pest algae in your tank. If your nitrates were really as high as that test says they are I'd expect to see your tank looking like a swamp. You have a 20 gallon with pretty easy to care for coral and not many fish. I'd do a big water change, like 10 gallons and test again. Do they drop by half right after, they should so if they don't your test kit is probably wrong. After a 50% water change some of your coral may close as it will probably change your alk in the tank but after a couple of days everything should open up.