You can go about i in many ways:
- you add things that consume Nitrates like macro algae (in the tank or even better in a refugium) or a large clam.
- Or you can add things that would contain denitirifying bacteria like more live rock or a denitrification chamber.
- Also, you can add a skimmer that would get rid of the disolved organic matter in the watter before it decays and eventually becomes nitrates.
The best thing is a combination of all of the above