so I was at Petco getting a dogbone and walked by the saltwater tanks. they had a number of very good sized mushrooms and other softies but the sand under them has a black sludge and sometimes its even on the frag plugs. I am thinking about getting getting one but is this an algae or something else? if algae i have heard i can do a peroxide dip but how is this done? if not alae what is it and what can be done?

