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The picture above demonstrates in a simple form the natural cycle of
organic substances (represented by the red arrows) being broken down in a pond with a biological
filter. This is called the Nitrogen cycle and in this case
demonstrates in particular what happens to fish food as it is digested
first by the fish and then the resulting faeces are then processed by
bacteria in the filter medium. The cartoon is one that featured in 'The Perfect Pond Detective Book 1: The biological balance' by Peter May, available from this site.

Biological filtration:

is a method of removing detritus/muck, organic compounds and specifically ammonia and nitrites from pond water by pumping pond water into a container and straining it through various foams, meshes or filtration media in which there is an established population of specific bacteria. These bacteria need plenty of oxygen to survive and they use it to digest organic compounds and a lot of the sediment that would otherwise lie decomposing in the same way on the bottom of the pond, although much more slowly. For small ponds there are two basic types of designs of filter that are built to work like this: Gravity filters and Pressure filters. 

UVCs or Ultraviolet Clarifiers are capable of making green pond water clear, but they do not make it clean. Without the help of a biological filter or a very healthy pond, UVCs used on their own are just making the ingredients for a pond polluting time bomb.

The term Ultraviolet Clarifier applies to a piece of pond equipment used to the green free floating microscopic that make fish pond water green. The unit is usually fitted on the hose line between the pump in the pond and before it enters a biological filtration unit. The water is pumped from the pond through the unit which contains an Ultraviolet lamp. The water is circulated around the lamp bulb and then proceeds on to the filtration unit. The effect this has on the water is that any algae present are severely damaged. It also causes the individual cells of algae to clump together and thus they are more easily filtered out of the water in the biological filtration medium and then digested by the bacteria that inhabit it.

If you have noticed foam forming around the bottom of the waterfall or in the biological filter, it may be a sign that the new pond and filtration system is not fully functional yet, but if has been lingering around for some time it may be indicative of larger problem serious enough to cause the death of your fish. A protein skimmer may be an expensive but it will be a sure-fire piece kit that helps you avoid  anything like this being a problem.

DOCs (Dissolved Organic Matter) the bane of the serious fishkeeper

As winter approaches and water temperatures cool, the seasonal change will exacerbate the problem that foam on the pond may be indicative of. One contributing factor may be excessive runoff from surrounding flowerbeds of soil sediment or accumulated debris around the pond, also trees shedding their leaves into the pond. Certain species like yew, oak, willow and elder can be particularly toxic, but also some innocuous small leaved types of Acer, although not toxic in themselves, break down very rapidly and can add quite a bit of organic matter to the water, which to a certain extent dissolves in it quite quickly. This organic matter is referred to with dread by Koi keepers as DOCs, 'dissolved organic compounds'. These are already present in the dampness of the soil and so can create an instant foam as soon as it splashes in.

OK, if you think pond skimmers are so important, what are they and why didn't someone tried to sell me one when I bought all the other kit for the pond?

Most ponds are in gardens and gardens have plants and shrubs and trees in them. These produce all manner of debris and detritus. In the Autumn you can cover your pond with a net to stop leaves from blowing in, but even so you will only stop the majority of leaves. Many hundreds of leaves will find their way in early or late on either before or after or under the net, despite any strict adherence to the mantra to make the pond site away from trees. The Oase Floating Lake SkimmerBesides there are hundreds of successful ponds that have only been properly landscaped into place when they have a ruffle of trees and shrubs around them. In spring and summer, pollen and flowers blow off the trees and in very late spring many evergreen shrubs and trees have a fall that sheds many of the old their old tired leaves.

If you had a swimming pool you would have a skimmer system built into the side of the pool sucking in the water and collecting all the floating debris on the pond before it sinks to the bottom. You folks iThe Oase poolside skimmer of the swimming pool type.n the USA have been building this sort of skimmer into the side of your ponds for years and you cant understand why I'm bothering to make any bones about it. Well, here in the UK it has always been something that we thought we didn't have to bother with, besides it requires quite a bit of forethought and plannning and real engineering i.e. building. There has always been the excuse that we wanted to keep our ponds looking natural and besides the skimmers would suck up the tadpoles and goodness knows what. 

Well the news is that pondlife friendly skimmers have been around for a long time, and now , even if you had forgotten to put a skimmer in that's no problem any more. You can put one anywhere in the pond and hide it with plants or float it out in an unobtrusive spot. Some sit on the bottom of the pond and by linking up to your present pump can be adjusted to any height to reach the surface. The floating type have are generally self powered with their own little motor, but with these you not only get all the muck and the pollen sucked gently off the surface, you get a blast of oxygenating bubbles below the surface too. The floating types dont have to be permanently their either. You could just set them up for that shed of pollen that say comes from Lime trees in the spring, or petals from Cherry trees in May. Anything that helps delay the inevitable day of the big pond clean out, has to be worth its weight in gold.

Bubble Bead filters or Powerbead filters are particularly suitable for Koi, especially where the pond has become a little bit overcrowded as the stock has grown, to help give clear and above all healthy water. They have the ability to remove the tiniest particles of detritus that can be forever in free-floating motion and not necessarily filtered out of the water in normal biofilters removing specks down to 10microns, but their main purpose is the breakdown of the pollutants in the water.