Question
moving to my present address this year, I inherited a large garden pond, 10,000 litres 10 x 8. The water was bad so I did a complete water change,restocking with fish and oxygenating plants but leaving some a large established water lily and some smaller ones. The water began to clear nicely and the ph was and still is fine. Over the past few weeks all the fish have started to die. Due to our dry weather wildlife has been drinking from the pond, and unfortunately a squirral and badger have had to be fished out after apparently falling in and drowning. Water seems to be going darker and slighly oily looking. What could be the cause of these continued fish deaths?

Answer
The dead squirrel and badger situation is very bad for water condition.  I would do another large water change, perhaps 60%.  There are only a few things that can harm fish.  They are bacteria, fungi, parasites, physical harm, toxins, electricity, organ failure, and long-term exposure to improper water conditions.  This should help as a troubleshooting guide for all future incidents.

For example, I would guess that there may be parasites from the dead animals, leading to bacterial infections.  The carcasses, depending on how much they decayed, could have even introduced a fungus.  There will also be an algal spurt, and the oils from the animals will create the murkiness you are seeing.

Just always make sure you dissect pond care into two distinct categories, fish health and water clarity.  They often do not go hand in hand.  I hope this helps you in your efforts.