Goldfish and Koi
Although it can be as simple as picking the fish that looks the prettiest, to the true enthusiast selecting a fish is much more involved. Considerations should be made when planning the pond as to what type of fish are desired. This plays a big role in determining the size of the pond and especially the filtration needs. Adequate filtration is very important when keeping fish.
Some people may choose from a few other fish species as well.
Keeping fish in your water garden does mean a little extra work. To a few this is a chore, but to many it is just part of keeping a beautiful and healthy pond.