The biggest trouble in the UK is Herons taking them and in one session an entire pond can be cleared. So make sure you don’t have shallow areas where a Heron can wade in and start fishing. If the water is deep enough a Heron wont be able to stand comfortably. Having said that another precaution is to string wire across the pond. It is not very attractive but if you’ve ever lost your precious fish to a bird you take any precaution you can.
Koi (Japanese for Carp) is a wonder fish if you have the space and time to look after them. You need good pumps and a good size pond with a depth of at least 1.5 meters so they can survive a long, cold winter. They were originally bred for food in China 2,500 years ago but over the centuries started being selectively bred for their amazing colours. In fact the humble goldfish was a originally a Prussian Carp until the Chinese started breeding it 1000 years ago. They can live up to 50 years and a very select few can cost thousands of pounds each.