Filters

External filters can either be sited beside the pond, returning clean water straight back into the pond, or the can be placed above a waterfall returning water via the water feature.

For a small to medium sized pond a single chamber type of filter would be used, which can handle flow rates from 2000 to 4500 lph, this is should be checked before purchasing. For a large pond , or if the pond is well stocked, you might want to consider a multi-chambered filter system.

Some filters incorporate an ultra-violet clarifier, this is normal sealed within a plastic chamber as the short wave ultra-violet light is harmful to humans and animals. This UV light kills any algae present in the water.

Function of the filter

For a filter to function effectively three different processes are preformed, these are:

Mechanical filtration
Biological filtration
Chemical filtration

Mechanical filtration is a process which physically removes solid particles from the water. Media commonly used for this process are: brushes, nylon mesh and filter matting or foam. When cleaning the filter matting do not use detergents as this will harm useful bacteria present in the filter tank.

Biological filtration is carried out by aerobic bacteria which are colonised on the surface of the bio-media used in the filter. This bacteria converts toxic ammonia (fish waste) into nitrites and then into nitrates. Anaerobic bateria then converts nitrates into free nitrogen.

Chemical filtration

This process removes wastes such as ammonia, through a process of absorbtion. Two common materials used are zeolite granules and activated carbon granules. Zeolite granules have the advantage that they can be recharged after they have become saturated, this can be done by washing them in a bucket of salted water. Carbon granules should be disposed of when they are spent.

It is advisable to monitor ammonia levels particularly in the summer months with a test kit.

    

To the left is a cross section of a typical pond filter, showing how the water flows through.

I used this basic design on my home made filter, as seen in the pictures on the right side of the page.

This is my filter setup, on the left is a hozelock uv 2200 filter. The one on the right is my home made filter, which is fed by a 4000 lph pump.



This single chambered unit cost me about 20 pounds to build. I used a piece of 20mm pipe for the filter supply, the spray effect was achieved by drilling about 15 small holes along the length and I used a small paint roller the block the end of the pipe. I used 38mm waste pipe for the exit pipe.



Three layers of filter pads were used and for bio media I cut up lots of 20mm lengths of plastic corrugated pipe.