I wouldn't worry so much about the nitrates as any level of ammonia in the tank. Ammonia can cause much more damage to your fish, even a low levels, gill burn, etc.. I would do a water change to get rid of it, and then as a precaution, use a double dose of "Prime" to make ammonia less harmful to the fish. For what it's worth, nitrate does little harm to freshwater fish except for extremely high levels. I have seen levels of nitrate at 160 in large, overcrowded freshwater tanks containing large fish with no obvious harm to them. Not that I recommend it. 
