A filter system serves two purposes for the pond, the first is to act
as a mechanical filter and the second is to act as a biological filter
Not all ponds will need a filter but in most cases where fish are the priority they will be required in order to keep water clarity and toxic levels below the fishes tolerance. With any system built for a pond the results will differ from one to another as there are so many factors and variables that can effect each pond. So what works for one pond may not work for another, if in doubt seek advice from a reputable supplier. Do not buy the smallest and cheapest you can find look around for the best product that will be easy to maintain and can run for a reasonable amount of time between cleaning, this is important for when you go on holiday.
Tip: Small compact filters will clog quickly as they have less media than larger ones and will require more maintenance. Look out for labels that say low maintenance and easy clean, as many are not.
The filter is a very important part of the pond system and you should spend time getting the best you can afford as at the end of the day you will be cleaning it, so the choice is whether you want to spend a lot of time cleaning a small filter or just the occasional clean of a larger filter.