Nitrates are the end product of the breakdown of ammonia. It would be more worrying if Nitrates were not present in the water as this would tend to suggest that the filter is not functioning correctly.
High levels of nitrates aren’t generally harmful to fish, but there are some very delicate species that will succumb to illness if exposed to high levels for a long period.
Levels of Nitrates can be reduced by changing water and increasing planting (which will absorb nitrates as fertilizer).
Proprietary kits can be purchased to measure nitrate levels.