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

1. GRAVITY FLOW 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.

2. PRESSURISED FILTERS 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 Violet 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.

The Bubble Bead Biofilter… it is a type of Pressure Filter. Only for use on larger ponds eg 100,000 litres. Not for the average pond keeper!!!
Important point…. The Bubble Bead Filter is both a mechanical (removes solids) and biofilter (removes dissolved ammonia and nitrites). It is not designed to be used on smaller garden ponds
A relative newcomer to the South African pond keeping scene is the highly efficient Bubble Bead biofilter. This filter works on the principle that the biomedium (ie the surfaces on which bacteria live and grow and carry out the action of converting dissolved ammonia ultimately to nitrates) is agitated and kept in suspension… what this simply means is that owing to the agitation or turbulence in the filter it becomes possible for a relatively small amount of biomedia to convert much higher quantities of nitrites and ammonia because the turbulence ensures that the bacteria on the surface of the biomedium always have a new and fresh sourch of ammonia or nitrites “to eat”. In other words the efficiency of these biofilters is extremely high and they can be sized to be used on ponds from small to very large.
The Bubble Bead filter works under pressure in a closed environment. They can be used on small ponds up to the very largest fish ponds (including commercial breeding ponds).
The Bubble Bead filter makes for a very compact and neat configuration and is the lowest maintenance pressure filter available. These filters are sized to handle up to 100,000 litres of pond water.
Adding to the extremely high efficiency of this biofilter is the fact that the blower not only increases pond aeration but assists in the backwash of accumulated debris on the bubble bead surfaces.
The efficiency of this pond filtration system is further enhanced by the use of a bio-booster containing the highly efficient biomedium Alfagrog.
Bubble Bead filters are low maintenance mechanical filters as well as highy biologically efficient biofilters due to the intense agitation involved… in chemical engineering terms the effective surface area for ammonia conversion is extremly high.

In summary Bubble Bead filters have the following advantages:

Occupy small area relative to filtration efficiency
Low maintenance operation
Highly efficient biological removal of ammoni and nitrites
Efeective mechanical solids removal filter system

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
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 + 10 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	Pressurised  Filter and UV Combinations
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.

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