I dont know if you are adding ferts or not, but be sure to test for nitrates before youadd add the ferts for the day. 20ppm nitrates are not all that high for a fairly well planted tank. I know that was after a water change. If you will do some investigating , I think you will find that nitrates , for for the most part , arent really a big problem. As far as the brown diatoms , well thats just part of a new tank cycling. They will go away just as they appeared. In time. clean out what you can , and give it a little time.