Probably need a bit more information about your setup, but a couple things come to mind.
- If you did a 50% water change yesterday, and your readings are still that high, chances are you were well over the 160 reading on the scale.
- I would probably continue with daily 20% water changes until you start to get the concentration under control.
- What filter do you have on this tank? Is it possible that the filter is undersized for the tank setup that you have?
- What types of plants do you have in this tank? Some are better than others at sucking up nitrates. Floaters come to mind, hornwort, etc.
- Any chance that your test kit is expired, or at least those used for the nitrate testing? I am guessing you probably have API kit - the newer ones show the expiration date on the bottle. I just ran into this yesterday where my GH kit was woefully expired. I couldn't figure out why it wasn't reading reliably, and then noticed how old it was. I swear it was expired when I bought it at the store because it was 2.5 years out of date.