Type of Filter required for a small fish pond?

[Naif]

We have a small fish pond in our house. It's circular and it's dimensions are as follows:

Depth: 9 Inch / 22 CM
Diameter: 26 Inch / 66 CM

We have a filter installed but clearly it doesn't seem the requirements. The water turns all green within a week so I want to know what I must look for in a filter?

 

[JohnHuff]

So your pond is approximately 20g. First things first.
- What kind of filter do you have there?
- What kind of fish, how many and size of each do you have?
- How often, how much do you feed them?
- Have you tested the water?
- How long have you had the pond?