Small dime size black irregular spots on rocks in 12″ – 24″ section of pond. Is this an algae or fungus? Used Algaefix…no luck after 24 hours. Can scrape off with finger nail, which then produces a little “black smoke” like vapor in water. Thanks for your help.
The stuff is most likely algae and the dusty stuff are the spores by which it multiplies. It may be better to clean it off with an oxygen based cleaner to disturb it less and reduce the distribution of the spores.