How to make biological pond filters, homemade filter, Alfagrog

Pondkeeper's How to...

How to make biological pond filters

How to make biological pond filters is a question often asked. Biological pond filters are easy to construct when you understand the principles of the biological pond filter itself

Reason for needing biological pond filters ......

In a lake or natural pond you will find relatively few fish. A natural water system can only cope with a certain fish density in view of limited food sources and biological capacity. In a pond there are almost always too many fish and that is why biological pond filters are absolutely essential.

How to build biological pond filters .....

biological pond filters can be simple boxes with biomedia inside

To be able to build biological pond filters you must know the pond volume. The volume of water together with the stocking density determine
the physical size and more importantly the amount of biomedia required to create effective biological pond filters.
Take the following information as a rough guide to size the box of the biological pond filters you want to make or build (you can always
add more but not less biomedia)

Biological pond filters - volume amount of biomedia needed per 250 gallons or 1,000 litres of pond water

Plastic tubes 1 inch diameter and 1 inch high (hollow): 8-10 gallons (ladies hair curlers will do the same job here)
Rough-surface plastic balls 1 inch diameter: 12-15 gallons
Lava rock: 2 gallons
Open cell foam: 5 gallons
Alfagrog sintered ceramic: 1/2 to 1 gallon
Multiply the above by 4.5 to convert to litres

These quantities of biomedia in biological pond filters are sufficient to handle the chemical waste in normally stocked garden ponds. Biomedia is of course the surface upon which the bacteria responsible for keeping the pond water pure reside. Obviously the greater the surface area in biological pond filters then the more bacteria you can have and the better biological pond filters will perform.

Basic decision for your biological pond filters ....

Decide if water is to flow UP through the biological pond filters (as in sketch by Peter J May above) or down. Upward flow through biological pond filters is preferred since solids would then tend to fall downwards.

A container of rough dimensions 3ft * 3ft * 2ft will allow biological pond filters to be made which will cater for all ponds up to 5,000 litres or 1,200 gallons so long as good biomedia is used. Smaller ponds can use a much smaller container.

Inlet and outlet pipes for biological pond filters ....

The outlet pipe diameter is very important because this type of biological pond filter operates under gravity conditions and if the outlet pipe is too small for the amount of water being pumped into the biological pond filters then the filter will overflow and your pond will empty - BEWARE.
For water flow rates up to about 100 gallons per hour a 1" pipe is OK. For higher flows through biological pond filters than this use 2" outlets. 2" will handle about 400 to 500 gallons per hour flow (2,000 litres/hour).
The inlet pipe should be around 1" diameter for flows up to 400 or 500 gallons per hour (up to about 2,000 litres per hour).

Internal components of biological pond filters ...

The above sketch shows the water if first pumped into a chamber containing brushes which are designed to trap leaves and other solid particles entering biological pond filters.

Such brushes make for very easy cleaning and offer little resistance to water flow. This chamber is not essential and it does make the building of biological pond filters more complicated.

Water then enters an empty chamber at the base of typical biological pond filters using the water upflow system. This chamber should ideally be fitted with a drain valve to be able to empty accumulated solid debris from the base of the biological pond filters - the sketch shows provision for this and it could just be a plug so long as well fitted.

Construct or build the biological pond filters chamber by incorporating a sturdy plastic mesh or sieve as you can see in Peter's sketch or drawing on which the biomedia can be placed.

Above the biomedia place open-celled foam or inert fibrous materiial to reduce the amount of solids that might find their way back into the pond.

Biological pond filters can be round, square, rectangular in shape.

Cleaning biological pond filters ...

To make cleaning of the biomedia in biological pond filters quick and easy place the biomedia into a net bag (used for oranges, onions and the like) before placing it into the pond filter. In this way the whole biomedia quantity can be removed in seconds, washed in a bucket of PONDWATER (not tapwater which contains chlorine) and replaced.

Biological pond filters using down flow - WARNING ...

Flow can also be in a downward direction in which case an overflow pipe must be built into the design of biological pond filters to prevent the pond being pumped dry if the biological pond filters exit becomes blocked.