Rust Coloured Gunge!!??

Question
Dear Jonathan
My mother has a small garden pond in which she has a number of standard goldfish.  The pond has been very healthy for 20 years but this year a rust coloured 'gunge' has appeared across the surface.  It is quite oily in texture and sits in clumps on the surface of the pond.  Do you have any ideas as to what is causing this and also how to treat it?
Many thanks
Jane (on behalf of Pam)

Answer
It could be one of three things:

1)  Algae, brown or a green variety that is dying.
2)  Mineral buildup from water supply.
3)  Micro-bacterial colonies.  (this would be a good thing)

They each have their own remedies, which are not interchangeable.
I would order a full water analysis and see what nutrients are overabundant.