Nitrates are the waste product of the bacteria, called nitrobacters, that remove nitrites. Nitrates are the end product of biological activity and organic breakdown.
Although nitrates are not as toxic as ammonia or nitrites, they must be monitored to avoid stressing the animals in your aquarium. Nitrates are toxic if their levels become too high and can cause many types of disease outbreaks. High nitrates can also be a source of algae problems. Nitrates will rise over time and can only be eliminated via water changes. Monthly tests are important, particularly when breeding fish, as young fish are more sensitive to nitrates than adult fish. Test monthly and keep levels low to ensure a healthy tank.