I've just installed a small prefabricated pond in my garden, no fish but
a few plants. Intention is just to attract wildlife and I saw a frog
dive in yesterday evening

Trouble is I am getting a greenish film on the surface, a net doesn't
pick up anything.

The pond gets full sun until early afternoon and hen light shade, it is
not directly overhung by trees.

Anyone any ideas as to the film and a possible remedy.

Geoff Lane