Anyway problem is I have a strange white oily film on the water surface, the
origin of which is baffling me

It's very common. I've heard it's caused by a bacteria. I've also see green
surface films, which are algae.

If you can, try increasing your surface turbulence. That will break up the
film so your filter can take care of it.

Also, make sure your nutrients are in balance. Sounds like you have plenty of
nitrate, but what about phosphate, potassium, traces, etc.?



Leigh

http://www.fortunecity.com/lavender/halloween/881/