A filter on a pond will benefit any fish, but is not a necessity providing there is a established growth of a variety of plants. It also important to bear in mind that the stocking levels & feeding must be kept to a minimum. Just compare the stocking level in a natural water (see any fish?) to that in an ornamental pond. This will give you an idea what the stocking level should be in an unfiltered pond. In any pond it is a good idea to have a small pump to run either a waterfall or fountain, this will help maintain oxygen levels on hot days.