[untamed]

Ammonia and Nitrite are both quite toxic and should always be zero once an aquarium has established necessary bacteria levels.

As the end product of the nitrate cycle, nitrate is much harder to get rid of. It is not nearly as toxic as ammonia and nitrite, but keeping nitrate as low as possible is a good goal as nitrate fuels growth of unwanted algae and impedes coral growth.

Methods to reduce nitrate are many, and include:
- growing/harvesting algae
- water changes
- Carbon dosing (vodka, vinegar, sugar, solid fuel balls...etc)
- Various types of denitration reactors
- Use of skimmers
- Controlled fish feeding

That's just off the top of my head. Next step for you is to research any/all of these methods and try to use any/all that work for you.