My 90 gal has been up and running since January. I have been doing weekly 50% water changes using Prime right along. I notice that some times, like today especially, I will get Brown debris all over my substrate and rocks. It looks almost like dirt. WHat is it? Is this normal? I usually have to vacuum and clean the rocks to get rid of it. WIll it go away on its own or do I need to keep cleaning it manually?