The short answer
Nitrate is the end product of your filter’s cycle — it converts toxic ammonia into nitrite and then into far less toxic nitrate, but nothing in the tank removes that nitrate. Only a water change does. If your nitrate is always high, it’s because more waste is going in (or accumulating) than your water changes are taking out. The fix is more or bigger changes, plus cutting the waste at source.
Why it builds up
Persistently high nitrate points to an imbalance:
- Not enough water changes — the number-one cause. Nitrate only leaves via changed water.
- Overstocking or overfeeding — more fish and more food means more waste converted to nitrate.
- High-nitrate tap water — some mains water already contains significant nitrate before it goes in. Test your tap water straight from the tap to check.
- Detritus build-up in the substrate slowly releasing waste.
How to bring it down
- Do a larger or more frequent water change — this is the direct lever. To drop from 80 to 20 ppm you may need several changes.
- Vacuum the substrate each time with a gravel cleaner to remove trapped waste.
- Feed less and reduce stocking if the tank is overloaded.
- Add fast-growing live plants, which consume nitrate as fertiliser.
Keeping it low
Once you’ve brought nitrate down, a consistent weekly change keeps it there. Test between changes to find the routine that holds nitrate in your target range. If it has been high for a very long time and pH is also low, read our old tank syndrome answer before making big changes. For target figures, see safe nitrate levels.