There are a few reasons why your nitrate might still be high after a water change. One reason could be that your tap water has high nitrates, and you’re not using a nitrate remover to remove them from your tank. Another possibility is that you have too many fish in your tank, and they’re producing too much waste for the nitrate levels to stay low. If you think this might be the case, consider doing a partial water change and adding more plants to your aquarium to help absorb the nitrates.