Why bother with a filtration system in your pond? After all, natural ponds and larger still waters don't have artificial filters, and they can generally support fish quite happily. Well, it's not quite as simple as that: a good filter is the only way to keep water crystal clear, while at the same time keep fish in tip-top condition.
The wild fish in our rivers and lakes don't always get it their own way, of course. They are at the behest of the elements, natural predators and, of course, the angler himself. However, in one important respect, they have a major advantage over their domesticated pond-dwelling cousins - they can swim off to pastures new if the water quality around them deteriorates.

Garden ponds are quite limited environments when contrasted with the natural vastness of rivers and lakes, and pond fish are completely at our mercy.

Before we go any further, we must clarify something: clear water is not necessarily clean water. Having the bottom of a lm (3ft) pond in clear, visible focus is very much a priority for us but fish don't generally care what they can or cannot see. For example, sight plays a tiny part in the life of our wild carp, which spend virtually all their life stirring up sediment into an impenetrable cloud in their search for small invertebrate food!

A large planted pond with few fish is indeed viable without a filter, but the 'safe' stocking rate for fish is 50-76cm (20-30in) length of fish per 4546 litres (1000 gal). At this rate you would be lucky to spot the fish between the plants, unless it was feeding time, or they were basking in the sun. A fountain, which increases the oxygen level in the water, would mean that the pond was capable of supporting a greater head of fish, but in nothing like the numbers made possible by even the simplest filtration package.

Filters, therefore, have a number of worthy applications: they allow us to have more, healthier fish; they help to clear the water and, more importantly, are essential in cleaning a pond. Specifically, filtration removes debris, while at the same time the inhospitable nitrogenous detritus caused by fish waste, the toxins caused by rotting plant material and general detritus are converted into less harmful substances.