Several kinds of aquatic animals can be added to water garden containers. None are required, but they can help create a balanced ecosystem. Goldfish are the first animal most people think of, but many small aquarium fish can live in an appropriate water garden container. Koi are a type of carp, and they grow over two feet long, so they won't do well in anything but a large container. Depending on the kind of fish and the amount of maintenance provided, the general rule for small fish is at least a gallon of water for each one is necessary if filters and pumps are used, and it is better if there is only one fish for every five to ten gallons if little maintenance will be available.
Just like in an aquarium, water quality must be maintained for good fish health. Chlorine and chloramine must be removed, either by waiting until they dissipate from the water or by adding an anti-chlorine product. Toxic waste products from fish are broken down by a variety of bacteria. Bacteria are required for fish health and do not come to a water garden on their own. They can be added from products purchased at the pet or water garden supply store. Fish will need to be fed except in large containers that have been established for such a long time that algae and other food sources have developed and are being maintained.
Snails can be added to water garden containers to help control algae and to consume organic matter. Frogs will tend to hop away from any container that they can get out of. Toads are not aquatic, except during breeding season and won't stay in a water garden container.