It's a strange fact that many people consider filtration on their ponds an unnecessary extra, or expensive luxury while very few people would argue the necessity of filtration in most aquaria. This possibly springs from a number of misconceptions, the main one being that natural ponds don't have filters so why should my garden pond need one? Natural ponds are in reality a fairly fleeting habitat. Plant growth and decay and the natural processes associated mean that over relatively short periods of time they become smaller and smaller, eventually becoming little more than a boggy piece of ground. While they still hold sufficient quantities of water to sustain a fish population this same plant growth uses up nitrate and other waste produced by the fish, as do huge natural bacterial and fungal populations found in the ponds silt layers so mechanical and biological filtration is redundant and predation, disease, oxygen levels and other factors restrict fish populations. In the generally far smaller confines of a garden pond this is not the case. A typically sized garden pond could be run without filtration, but the numbers of fish you would be able to keep would be far, far smaller than if filtration is installed and the pond itself would just like it's bigger, natural counter-part rapidly silt up. The only real reason for not installing filtration in your garden pond is that it is to be for wildlife or plants with no fish. There are a number of different types of pond filtration available, as well as supplementary measures to help ensure that your pond water stays clear and safe for your fish. The type suited to your pond depends on several factors including the volume of the pond, the type of fish you plan to keep, whether you are fitting the filter as part of the pond construction process or as a later addition as well as more obvious constraints like budget, space and aesthetics.A typical garden pond containing goldfish, orfe and other similarly sized fish can usually be filtered by an appropriately sized combination biological and ultra violet,(UV)filtration system. These can be bought in a range of sizes suitable for your ponds volume and it is usually advisable to purchase one that is rated for use on a pond around 10% larger than your own - there really is no such things as too much filtration.Where space is more limited then a compact pressurised filter, which can also be bought with a built in UV may be the best option, but they require more regular maintenance as a result of their smaller capacity. Many of these come with a 'self cleaning' option which can help with this however.Koi ponds are without doubt the most expensive ponds in every aspect, from construction, to filtration and livestock. Koi require huge volumes of water, and considerable depth with 1.2m,(4ft) being the minimum - these are fish capable of growing in excess of 1metre and living decades if given the right conditions. As a result the filtration required for them is similarly expensive and therefore needs serious planning. Bottom drain connected sand pressure filters, huge biological filters and protein skimmers are just a few of the methods used.