I have a 55 gallon tank I bought from Craigslist and cycled it. The ph is around 6.6, 0 ammonia, 0 nitrates, but my nitrites are around 80ppm. I've done a thorough cleaning of the tank and gravel. I've done a lot of water changes as well but I can't seem to get the nitrites down. Any advice? Are my fish in danger?