Pond Filtration

The objective of filtration and aeration is to cleanse and oxygenate your water so it can be recirculated and used many times before it needs to be replaced (in small amounts). If you did not filter and aerate your water it would quickly become a smelly and lethal body of water in such a murky condition that nothing could live in it but dangerous bacteria.

In water where there is visible debris you can remove these by passing the water through a mesh in which the hole size is smaller than the debris. Only the tiniest pieces of dirt will be left. If you then pass this water through even finer material, less debris would be left and the water will look a lot cleaner. This is mechanical filtration. It prevents detritus from passing through a mechanical barrier that can be made from several materials such as nylon wool, filter pads, gravel, blue furnace filters, quilt batting and so on. Periodically, you remove the filter material and clean or replace it. Settlement tanks can be used in this process and below are 2 diagrams of mechanical filtration with and without a settlement chamber.

Three stage filter without settlement chamber(s).	Two stage filter with two settlement tanks.
Chambers such as these use the immense surface area of all the stones combined for algae to grow on.

Mechanical filtration does not remove dangerous chemicals and gases from the water though. This can be achieved by certain beneficial bacteria. These organisms live on rocks, gravel, sand, pot scrubbers, plastic forks, most surfaces. They are aerobic, thus need plenty of oxygen to survive and build up colonies. Along with your plants, this is what is known as biological filtration. Below is a chart showing what the biological filtration does.

The nitrogen cycle demonstrates what occurs when the wastes of assimilation (the incorporation of food by pants and animals) are converted by bacterial oxidation [the leftmost item] to nitrogenous compounds.

Ammonia (NH) is highly poisonous to fish and must be kept out of a healthy pond or tank. A good biological filter will achieve this. Even small amounts of ammonia can prove fatal to fish. It destroys the delicate mucous membrance of the skin and then penetrates the internal tissues.

Ammonia is created as a result of the breakdown of nitrogenous products, such as fecal matter and proteins. When it combines with water is produces ammonium (NH4), which is less harmful, but is still potentially lethal. This is because the ph value influences the amount of ammonia that a volume of water contains. At low ph values most of the ammonia will be converted to ammonium, but at highter ph values (above 7) this does not happen; in fact it increases the ammonia levels. Ammonia is utilized by certain plant life as a form of food, and it is also a food to anaerobic (non oxygen breathing) organizims. These bacteria (Nitrosomonas) convert ammonia to nitrite (NO2) which is only a little less harmful than ammonia. However, other bacteria (Nitrobacter) convert nitrite into nitrate (NO3). This can be tolerated much better by the fish and is utilized by plants (usually algae) as a food source.

This is why Plants are so necessary in a healty pond!

Below is a diagram illustrating the conversion of ammonian into nitrites then nitrates and a diagram of a skimmer that you can build for your pond.

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