We have had our pond for 6 years it is about 1600 gallons with a 8 foot stream with a very good density of vegetation. The last 2 years the rocks in the pond have a very dark thin layer of algae I think on them I have seen other ponds that don't have this discoloration the stream stays very clean we use green clean on it and we use pond balance in the pond but can' control the dark coloring algae on the mail pond any suggestions and Thanks in advance.