Nitrates in themselve don't pose a threat to your fish, but they are a sign of disolved organic coumpounds in the water, wich are detrimental to your fish, so ideally you want to keep nitrate as low as possible via water changes (chemicals won't help with the DOC's). So, short answer is, nitrates under 20 is good, even 0 is good unless you have plants, then you want to maintain it at 10-15 via dosing (adding pure nitrate doesn't contribute to DOCs).
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