This is probably the most difficult type of pond to design and build because there is much to consider for fish. What sort of fish do you want to keep? Goldfish are quite hardy and can live in most ponds, but other fish, such as larger Koi Carp, need a fairly deep pond – at least 4feet deep (some say 6feet). Either way, for keeping fish, you need to have it at least 18inches deep for the Winter so that the fish have somewhere to live when the pond freezes over. You will need to keep a hole in the ice if it freezes over in any case when you keep fish, in order to stop toxic gases building up in the water which could kill the fish. You will also need to keep the water quality good if you intend to keep fish. A filter system is almost obligatory. The surface area of the pond should be enough to accommodate the number/ size of  fish you intend to keep. The bigger the surface area, the more Oxygen exchange and the more/ bigger fish you can keep. Don’t forget that the fish will grow!