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 I swear it was expired when I bought it at the store because it was 2.5 years out of date.
I have a fluval c3 on my tank. My tank is 5.5 gallons, and I know its harder to keep a solid steady param in a smaller tank.
I just bought my API kit last month after the other one had expired, so it's not expired. I have blyxa, moneywort, fissiden fontanus, java fern, salvinia cucullata, dhg, wisteria, colorata I think, and a couple unknowns.
I'm using azoo plant grower bed as a substrate, have manzanita in it, sponge pre filter, ph is 6.4, ammonia is 0 nitrite is 0.