My tank has been up for about 2 years now and some of my sps corals seem to be dying. I thought it was an Alk problem but found elevated levels of Nitrates. My nitrates have always been low to the point I stopped checking them. Where are they coming from. I had a filter sponge that I've used for about a year now and clean it weekly. Could this be holding the nitrates? I just changed it to a new one. I have some macro in my sump and I know that is supposed to help keep levels down. What is my best way to lower levels quickly to prevent further coral damage?