Last year we replaced the pond liner and all the water. Since about July this year we have had a microscopic brown layer on the pond surface that now covers the entire pond. The water itself is not coloured other than we use a black organic dye but it’s definitely not caused by tannins. The pond has no fish just a bulrush and iris in containers with specialist pond compost. It’s impossible to scoop out the brown stuff even when removing duckweed with a net which I thought might act as a filter. When I do this the pond appears ok for an hour and then the brown layer returns. I thought the cause might be brown algae but l am not sure.