Water change in a 10 gallon tank

Hello all. I just check the nitrate, nitrite and amonnia in my 10 gallon saltwater tank and the nitrate and nitrite are ok but the ammonia is 0.25. It's been like 20 days that i haven't made a water change. How does it work? Are the nitrate and notrite parameters that really change ? Do i need to check others parameters ? Thanks a lot