"Dr Engelbert Buxbaum" wrote in message
...

Could be a mixture of microorganisms living at the air/water interface.
The layer is very thin, in the order of the wavelength of visible light.
Hence the rainbow effect (diffraction).

In itself this is harmless, but it might indicate that the water is
quite stagnant, limiting the dissolution of air oxygen in the water.

By the way, if you have access to a microscope, float a cover glas on
the water surface for 10 min or so, then observe the organisms (using
tank water as mounting medium). Cilliates, amoebae, bacteria...

great idea. i'll make a slide immediately.