The Different Types Of Pond Filters And Biofilters

It’s important before we start on this article to make these vital points. When we talk about pond filters and ponds in general we are not, first and foremost, talking about solids removal. We are talking about biological purification of fish pond water so that the fish do not poison themselves to death. This is critical to understand. I am not sure how this word filter came into existence with relevance to pond keeping but it has caused more confusion and damage and waste of money than anything else in pond keeping.

Let me be blunt… A biofilter is for keeping fish alive by removing poisons that build up in pond water. It has nothing whatsoever to do with filtering out solids as a PRIMARY function. The fact that it might remove some solids is a by the way incident… the fish don’t mind at all that solids settle to the bottom of a pond. Take a look at the bottom of all natural ponds and rivers and lakes.

These products will help with filtration: Alfagrog, Viresco, UV lighting


Everybody with a swimming pool is aware of what a traditional filter does… a filter is a device to separate liquids from solids held in suspension as the water/solids mix enters the filter.

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So a mixture of solids suspended in water is fed through a conventional filter. The solids are removed on a closely packed substrate bed and this is normally river sand. Hence the generic name for swimming pool filters as “sand filters”. This type of filter bed is very efficient in removing solids because of the very fine pathways in the bed of sand… water can pass under pressure but the solids are just too big to get past the sand particles.

IMPORTANT: Now this is NOT what is meant when the word filter is used in the context of pond keeping.

In fact the word to use always is BIOFILTER and not filter… why?

Pond BiofilterFilter refers to removal of solids from a suspension of solids in water. While this might to some extent be true in a pond filtration system the actual meaning of filtration in a pond circulation system does NOT refer to solids removal alone. The correct term to focus on is BIOFILTRATION not filtration.

Biolfiltration In A Fish Pond

What’s far more important in a pond is to purify the water that becomes contaminated by the natural biological processes of fish swimming and eating in the pond water… this is what a BIOFILTER does. Fish in a pond couldn’t care less about solids that settle in the pond… in fact they welcome these solids since they become a breeding ground for midge larva for example and these become food for the pond fish.

However without biofiltration installed the fish’s life is at risk through ammonia and nitrite poisoning. Ammonia and nitrites are by products of a fish’s metabolic processes. The nitrogenous compounds are removed by naturally occurring bacteria which become concentrated in a well designed biofilter.

So quite simply a biofilter is designed and operated PRIMARILY to remove dissolved (ie not suspended solids) nitrogen based chemical compounds (Nitrites) that if they were allowed to build up to higher concentrations would poison the fish in the pond.

Let me say this in a single sentence because it is important… a biofilter is PRIMARILY installed to purify pond water and not to remove solids. If the biofilter also removes some solids in the process then this is a bonus.

In a natural pond (or river for that matter) the natural biofilters include all sunken debris such as stones, trees and the like. Bacteria treat these sunken items as home by clinging to their surfaces and as a result significant biofilter capacity incurs in any natural environment. These natural biofilters and relatively low stock densities of fish and other wildlife allow all the creatures in the pond to live in harmony and equilibrium with their surroundings.

Different Filters Commonly used in Pond Systems…

I am always concerned when I write about larger, more expensive and sophisticated types of pond filter (biofilter) discussed in this article. The vast majority of garden fish ponds need unsophisticated simple to install box type (or cylindrical shaped) biofilters. The discussion of bubble bead and other pressure filters here is aimed at the more ambitious pond keeper who wants to install an efficient but larger pond. Do NOT be discouraged from installing a garden fish pond by this discussion since it will not apply to the vast majority of garden fish pond situations.

CRITICALLY IMPORTANT: My job is not to persuade you to spend more than you need on a fish pond biofilter. I will say this again… the vast majority of smaller garden fish ponds need a very simple low cost Filter Box system (round or rectangular preferably with Alfagrog filter medium and perhaps a couple of sponges) combined with a suitably sized UV light. Do not be persuaded to invest in larger more sophisticated systems unless you understand exactly why you need these. Having said that there are good reasons to consider more elaborate and sophisticated systems if you want to truly enjoy and grow your fish keeping hobby. Ultimately you need a low maintenance effective pond filtration system that keeps your fish alive and prevents your pond water going green.

Simple box type static biofilters… these are the most widely used and simple types of pond filters (I should say pond biofilters). They come in many shapes and sizes and the innards of which also vary dramatically from stones and sponges to wire brushes and Japanese Matting. Whatever the filter medium used (correct term is biomedium or biomedia for plural version) the principle is the same… the biomedium is used as a place on which the bacteria settle themselves in order to biologically purify the water flowing continuously over the biomedia surface by converting nitrogen impurities (called Ammonia and Nitrites) to the fish-safer form of nitrogen chemical which we call Nitrates.

In summary a biofilter can be any type of medium on which bacteria can accumulate and so long as there is enough of this substrate and so long as the pond water contains oxygen dissolved in it then the biofilter will purify the fish waste that becomes dissolved in the pond water. Even submerged rocks act as biofilters. It’s as simple as that. Of course the more fish in a pond or the more food fed then the bigger must be the capacity of the biofilter in order to house sufficient numbers of bacteria to convert the contaminants to nitrate forms which remain dissolved in pond water and which are a much lower threat to fish health.

Having said any type of medium can be used I need to clarify this… if you want lots of bacteria then there must be lots of surface on which the bacteria can live and breed. Now these bacteria are very very small and can inhabit tiny surface indentations so a rough porous type of surface is the ideal medium to use in a biofilter. Smooth pebbles or plastic balls are bad examples of biomedia because they have small specific surface area… this means in practice that if plastic rings, cylinders, hair curlers and any other type of plastic medium is used in a biofilter then the size of the box to hold enough area (space if you like for the bacteria to live on ) must be significantly larger than a box holding biomedium with high specific area.

Alfagrog is a great example of a biomedium… it is a very rough and very porous sintered ceramic product. This means it is open and lightweight and provides massive amounts of tiny spaces in which bacteria can live and thrive.

As you feed your fish more and as they grow bigger then you may well need to increase the holding capacity of your biofilter… this is easily done in most cases by adding a few litres of Alfagrog once a year. Do NOT discard the old Alfagrog or let it dry out.

The Bubble Bead Biofilter… It is a type of Pressure Filter. For use on larger ponds eg 10,000 litres. If you have a pond of 5,000 litres and up the Bubblebead would be the way to go.

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 (i.e. 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 source 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 15,000 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 highly biologically efficient biofilters due to the intense agitation involved… in chemical engineering terms the effective surface area for ammonia conversion is extremely 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 ammonia and nitrites
Effective mechanical solids removal filter system

IMPORTANT: these filters are not for use on garden ponds of less than 5,000 litres (i.e. the vast majority of garden ponds). The vast majority of garden pond keepers will never have a use for these filters. They are aimed at larger ponds installed by koi keepers where 5,000 litres is considered small.

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