When your doing these water changes make sure to remove as much detritus as you can as well. Blast the rocks with a power head, stir up or vacuum your sand bed on occasion, clean any sort of mechanical filtration on a regular basis etc. Nitrates don't necessarily just show up. There's something fueling it, whether it's leftover food, an abundance of poop etc. If you get into the habit of removing this, then usually you won't have problems with that. GL.