What filter is best for my size tank?

Ok I have a 21 litre tank (Hagen Marina Cool 21 Goldfish Aquarium Set) I have a filter that came with it which is a Marina i25 but i dont think it does a very good job and sucking things up so does anyone have any idea what other fitler might be good for this tank i would like one i can put completely in the tank and Im planning on getting some white cloud mountain fish they only grow to be 1.5 inches in length but i dont want an undergravel filter. Ive been looking at the ebay shops i would like something thats easy to maintain but theres so many different filters they are all confusing me with their pros and cons :P Thanks