Right now in my tank a 55g established tank of about 8 years. While testing I found that my nitrates are exceeding the maximum of my test kit of 200ppm while all other parameters are pretty normal. Not too long ago my dkh was down to around 2 and I slowly brought it back up to it's normal range. The tank doesn't seem to have any negative effects and the live stock looks healthy. Should I be concerned? In the past 3 weeks I've done water changes to try and lower it but it doesn't change much. The test is new and not expired. In total over done probably a 25g water change. The tank does have a sump of about 7 gallons of water volume been in about a month. I regularly use and replace carbon. Skimmer is working properly, and I supplement the bio filter with additive monthly.