How to choose a filter

When selecting a filter for your pond you must first ask yourself two important questions:

What fish are you going to keep? Koi carp have a much heavier "waste load" than goldfish.
How long do you want to keep the filter? Goldfish grow approximately 1-2 inches a year, Koi can grow 4-6 inches a year. A borderline filter that can just cope the first year or two will be unable to keep the water clean as the fish grow and their waste levels increase.

Once you have decided, you should consider the following points before making a final decision:

Mechanical filtration - How will the filter deal with heavy solids (faeces, leaves etc).
Biological filtration - This breaks down ammonia and nitrite (fish waste). What media is in the filter? How much surface area has it got?
Efficiency - New filters are usually quite efficient. What will happen after several weeks? Will the filter begin to block? Will dead spots occur? Is there a media grid to aid flow through all the filter?
Cleaning/maintenance - How often will the filter require cleaning? How easy is it? Is there a waste valve to simplify regular maintenance?
UV (ultra violet light) - Ensure the UV light is the correct size for your pond. A half clear pond is still green!

All biological filters require a minimum of 4-6 weeks to mature and up to 18 months to mature fully.

Regularly check water daily for the first 6 weeks and at least weekly thereafter (ammonia, nitrite, and pH).

A clear pond is not a clean pond.