I have a fish pond with two butterfly koi, shubunkin and goldfish. Over the last month six fish have died plus another today, all with the same sumptons of, a small patch at the end of their body, then when its to late to save them, half of their body appears to be covered in what looks like a thin layer of green algae. The attached picture is not very clear but I am going to take a better one tomorrow motning - has anyone any ideas before they all die.