Post a pic of the pond and let us take a look. It could be a real thick plankton bloom. That is a lot easier to deal with than DW/WM.

The top pic is a plankton bloom with about 16in visa. The second pic is a later stage plankton bloom with a red euglena bloom on top and it is thicker and like a film on top. Plankton blooms can get much thicker than these and look like a sewage pond all gummed up.