The Tench is a hardy fish; one or two in larger pond would not go amiss
They are well known as pond cleaners, they spend most of their time at the bottom looking for food in the silt such as anything uneaten by the other fish.
Because they spend a lot time at the bottom it would be very rare to see them, but they do a good job of cleaning up the pond of unwanted rotting food.
The Gudgeon comes in many types and is a very small and hardy fish that grows to about 8 inches long, and can tolerate any weather conditions; this makes them ideal for any size ponds.
It would be a good idea to have a few in your pond as they like to live in small groups.
Once in your pond they tend to disappear to the bottom where they love to feed off waste, insects and larvae, this helps to keep the pond clean.
To make sure they are well fed, use some food that sinks from time to time, this keeps them healthy to carry on the good work they do.