been thinking about this after posting thread in water quality . water is cloudy and i filter out a LOT of green particles daily without satisfactory improvement . pond is 12x20 [ all distances ''rough'' not measured ] long dimension runs east to west . there is a 50' tree about 5' to east , another about 45' to west , house about 7' to north and big [ about 100x400 ] pond about 15' to south . small pond is shaded for much of the day so why the algae ''problem '' . the big pond is very clear , at least in shallow area near shore , and yet gets full sun most of the time . also no one tends the pond, feeding fish , filtering , or checking water quality for example . no one feeds fish and they get along fine and some are good sized . the few carp do congregate under our mulberry tree which over hangs big pond and feast on fallen berries this time of year but no one is eeeding them intentionally . so , why algae in our shaded little pond and zero in sunny big pond . . peace