Posted: 13/02/2015 at 10:39
If none of the above suggestions are the cause, you could perhaps get a sample of your pond water analysed.
Failing that, it may be worth contacting your local water authority to see if they have any advice. Whilst it isn't within their remit to help you out with your pond, they may well be able to give you some pointers. Likewise with your local environmental dept.
The above suggestions may sound a little weird but most public service companies like to be seen as "customer oriented" or "user friendly" these days. Always worth a try anyway