Do you need a pump and filter?
The first question is do you need a pond filter or pump on your pond. The simple answer is no you don’t need a pump and filter but…(there’s always a but ?) you will probably want one.
It all depends on what you want from you pond if you want to have clear water so you can see the fish and other life forms in your pond you will need to install a pump and filter. If on the other hand you simply want it to be a natural pond you could maybe get away with not having a pump and filter. I say maybe because even to create it as a natural pond you probably should still have some kind of water movement and filtration process otherwise you are going to have a pool of stagnant water in your garden which is going to start smelling and probably not be the attractive focal point you desire.
The main reason a pump and filter is essential
The other thing you need to consider is whether you are going to be adding fish food to the pond in-order to feed the fish. The reason I ask this is because any pond will be able to support a small number of fish naturally i.e. the pond will be able to attract enough food and be able to cope with the fish waste produced of a certain number of fish without the water quality reaching dangerous levels. Obviously this number is different for every pond depending on various factors such as size, amount of plants, location etc. The problem comes when you add more fish to the pond than it can naturally cope with and you keep them alive by adding fish food. The result of this is that more fish waste is produced than the pond can naturally cope with, leading to an imbalance in the water quality (see pond balance for details about balancing your pond). This imbalance will then lead to you having water that is unacceptable for the fish and other life to be able to survive in (i.e. nitrate, nitrite or ammonia too high). The only way to balance this out is to install a pump and filter to help the pond maintain the perfect balance and cancel out the effect of you adding additional food.
Types of filter
There are a number of filter types including Pressurised pond filters, Gravity fed pond filters, in-pond filters, bottom drain ponds. As to which is the most appropriate, it will all depend on the size and type of pond. As a general rule I would say that a pond with a bottom drain is always the best method but totally un-necessary in many cases especially for smaller ponds.
As for the rest I personally would go for a pressurised filter and matching pump for small to medium sized ponds and a gravity return filter for larger ponds or highly stocked ponds! Anyway use the links to read all about each type of filter and see which is most suitable for your particular pond.