Have you had the water tested?

Firstly thank you for replying.......Yes water tested x 2, fish bacteria killer also added ....

Sorry for late reply, hope they are all not dead. Have you checked your Ph? Mine crashed once, and I lost several fish. Please check Ph. If it's high OR low, you can add baking soda to stabilize it, although that's a short term fix. Long term introduce some crushed oyster/clam shells into the system and that will stabilize the Ph. Good luck, please let us know how it's going.

Thank you to all who responded. We had another largish fish die. Think the rest are now healthy(ish). Water level, PH, bacteria were all OK in the pond. The last one died after my husband cleaned some of the debris (leaves etc) that had settled on the bottom of the pond. Really don't know what the problem was, only thing it might have been is that we had very hot weather and perhaps there was not enough O2 in the pond. We have since put in more air outlets. It is a big pond, so hopefully they will all remain healthy. Again thank you to all who responded. It was very much appreciated.

Again, thank you. We have put a bubbler in and we now know to clean the filters more regularly. We have had the pond for about 5 years and this is the first time we have had fish die. However, having taken advice as you describe the remaining fish all look to remain healthy and are feeding well.