Rust coloured gunge

Question
QUESTION: Dear Raf
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: Dear Jane,

Thank you for your mail.
Normally when something like this happens sudden it mostly comes due to some changes.
Like trees added, very warm summer etc.
At this stage it is hard to say without samples or pictures.
A possible way to reduce it is by pumping the bottom areawater away.

Good luck and please if possible sent me a picture.

Kind regards,
Raf

---------- FOLLOW-UP ----------

QUESTION: Hi Raf
Thank you for the speedy reply and the information.  Since contacting you we think that we may have identified the cause - my mother had a treatment applied to her lawn at the beginning of May.  The day was quite windy and we think that some of the treatment (a powder) may well have blown into the pond.  With each day the pond is becoming clearer and so we will continue to monitor the progress.
Once again, many thanks for your help
Kind regards
Jane

Answer
Dear Jane,

That is good to hear.
However, it is indeed dangerous for loosing fish.
So hope everything go's well, just the advice to try to get rid of the debris as it will tend to build up in the liver of the fish.

Good luck and have a good pondseason!

Warm regards,
Raf