There is a brown muck on the filter and fake plant in my 2 month old, 10 gallon tank. Last water change was a 40% 2 days ago. I have not touched my whisper 2-10i filter since set up a little over 2 months ago but I'm going to replace the carbon bag tomorrow. Temperature usually ranges between 76-78 F. The tank gets a good amount of natural sunlight and the tank lights stay on for 10 hours a day. Could it be lighting or something else? I really appreciate any feedback since I'm a newbie, so don't hold back.