I have just had a lady ring with a problem. She set up a large pond about 4 months ago. It is lined with polythene, 400cm deep and about 30,000 litres. It had about 100 goldfish in it (largest was 8"). Recently they have been dying and she has lost 45 over 3 weeks. Their tails appear to be shredded. If it is finrot, is there some way to treat a pond that size that doesn't include medicating such a large volume of water?
She has tested the pH and nitrites and both are good.
There is a fine layer of pollen on the surface and she wondered if this was causing a problem. Fish are at the surface but not gasping (although that is probably normal at the moment as it is warm and sunny). There is a small fountain at one end and an overflow, so it has had plenty of fresh water into it over the past month thanks to our heavy rainfall.
Sorry about the mix of imperial and metric but it is how she gave them to me 8)
Any ideas please? They are off to Aussie tomorrow and she is concerned about more dying while they are away.