I have a 55 gallon tank that has been healthy and well establised (over 10 years), over the past few months I have seen the growth of dark gray feathery like invader. Not sure if it is a fungus or what. So far fish dont seem to be bothered by it, but I am. I removed all artificial plants, cleaned filer (Fluval ?605) changed all components of filter ( I know, should do all at once but thought I would try) now its growing pretty quickly on the gravel. Any ideas what it may be or how to treat it. I would hate to have to change the gravel. Will try to upload photo.