I have been keeping saltwater for awhile and now have to catch back up with my freshwater info. the tank is a 55g with 2 emperor 400 filters on it. the nitrates are up to at least 160ppm from not having the media or water changed in I dont know how long. tank wasnt in my care till recently. just yesterday i did a 75% change on it and the tests were done after that. what should I do to bring the nitrates back down? another water change? new filter media?