Can someone help me as to how to control algae that is growing in the pond, especially on the pond liner? The water is turning green and on the waterfall, it keeps turning black (algae, moss?) I wash it off with my hose but it comes right back. I have 4 fish in my pond (Koi) and 3 turtles. I have a pretty good filter and have several pond plants. Can someone please help me? I am new to this pond stuff.