Ponds, Pumps & Filters...

How to Select a Fish Pond BioFilter

Why You Need a Biofilter and How it Works

It's important to realize that a biofilter is needed to purify the pond water ... fish pollute the water through natural metabolism. These nitrogen based toxins reach lethal levels if not removed A filter is also useful to remove suspended solids from the water.

If your pond is small to medium and you do not intend to stock with expensive fish then there is little need to overspend on a biofilter with all the bells and whistles and hype. Price is not the determinant of how good the filter is. Your choice will be between a gravity (box type) filter and a pressurized one. No matter which kind you select it is the biomedia that is important.

Two kinds of biofilters to suit any pond

UZ Gravity Biofilters


Low cost gravity flow biofilters are used in any size pond. The picture is of a range gravity biofilters available. These filters can be supplied with two types of biomedia... either (1) Bio Balls which are 40mm square plastic components specially made for use in low pressure biofilter systems or (2) Alfagrog which is a high efficiency biomedia developed in the UK for the fish farming industry. Alfagrog is a sintered ceramic material which means it is extremely hard and porous yet does not break up.
Please see below for specifications. These units can be piped up in parallel for very large systems making them suitable for any sized pond.  With the very high electricity cost these days ponds operating with sand filters should seriously consider changing to this type of filter ... such a change will save many thousands of Rands over the life of a pond.

pressure biofilter


Pressurized filters (see image left) can be easily hidden or located some distance from the pond itself. Cleaning is easily accomplished by using the nozzles available (see picture). Pressure biofilters need an extra powerful pump head to push the water through the system.
Pressure filters are almost always sold with an Ultra Violert Light combined with the pressure filter itself. This si to ensure crystal clear water is achieved and to make installation of the combo unit easier.
Follow the recommendations in the pump selection page to specify a pump and if any doubt go at least one size larger.


The principles behind correct specification are the same in all filter types.

You Must Know Your Pond Volume ... That's All Really

To specify an appropriate biofilter it is important to know the volume of water in a pond to about 80% accuracy. A filter is specified in terms of what capacity it can handle when the pond is stocked normally. The greater the mass of fish in a pond the greater the level of pollutants (because more food is fed) so a larger filter would be used.

How to select a biofilter

A filter will claim to handle or purify up to 5,000 litres of pond water. The claim will be based upon qualifications such as fish stock density, amount of food fed and so on. If you can afford it always go slightly bigger to get better performance.

A biofilter will NOT clear a murky pond caused by suspended algae without help from an UVC (Ultra Violet Clarifier). Many modern biofilters have a built-in UVC. This is a good choice of filter to buy.

Available Biofilters and Specifications

Pond Volume Gravity Filter Recommended (see image above): These units come with stepped hose fittings meaning any size pipe can be attached easily.
Up to 2,500 litres 5 litre Biofilter with Alfagrog
2,500 to 5,000 litres 10 litre Biofilter with Alfagrog
5,000 to 10,000 15 litre Biofilter with Alfagrog
10,000 to 20,000 litres 25 litre Biofilter with Alfagrog
20,000 to 30,000 litres 50 litre Biofilter with Alfagrog
30,000 to 50,000 litres 2 x 50 litre with Alfagrog
Pond Volume Gravity Filter + UVC Combo Recommended
Up to 2,500 litres 8 Watts UVC + 5 litre Biofilter incl Alfagrog
2,500 to 7,500 litres 15 Watts UVC + 5 litre Biofilter incl Alfagrog
It is the wattage of the UV that determines capacity of the Combo system.  If your system is larger than the above you can pipe up 2 units in parallel or opt for a separate UVC and one of the above biofilters. UVC lamps MUST be changed annually.
Pond Volume Pressure Filter Recommended incl UVC (see image above)
Up to 10,000 litres Pressurised pond bio filter with an UVC built in to control algae in suspension which causes green murky water. Filter can be buried to hide it away. Suitable for ponds up to 10,000 litres capacity.