Is this an indoor or outdoor pond? How big is it? Is is a concrete bottom or soil? It is not as simple to turn a fountain into a pond by just adding fish. You cannot just add clorine to clear up the water. Fish need a very balanced environment and can get killed quite easy with roach and termite sprays used indoors and ant killers and lawn fertilizers outdoors. You have to put in filters and water treatment systems. Your fountain pump (if it is a submerssible model) will probably need to have an input filter system added. If you have an external pump system then you need to add an input filter, external filter system. You need to include aquatic plants for cover, oxygen, shade and protection.
I would probably start with goldfish. Not the normal "gold" goldfish that you find at walmart or k-mart. But fish from a pond or aquarium supply with outdoor pond supplies. Goldfish from one of these suppliers come in quite a wide range of colors and are cheap ($0.50-2 each). Get 2-3 dozen per 500 gallons, many will die within days and weeks. They take to first setup ponds better than other species and help build up the environment. They are also a good learning experiment without a major investment in expensive Koi. Koi is just a bigger breed of goldfish or carp. Once you have an extablished pond, you can add the Koi if it is large enough. The Koi will eat the other fish in the pond eventually. Koi can grow to be quite large. So go easy on them. If the pond is outdoors you will have to prevent racoons, skunks, cats, Herons, Gulls and other predatators from eating them all before they have a chance to grow up. Deeper ponds offer more protection from predators along with the aquatic plants.