How Do BioFilters Work To Clean & Purify the Pond Water
In almost every garden fish pond there are far too many goldfish or koi than
nature ever intended to put into such a small space and volume of water. Nature
also never intended to feed fish with Bradshaws wonderful high protein (high
Nitrogen) floating pellets or sticks. Nature also generally allows a pond to
become a bit untidy with things like sunken trees, lots of rocks, flowing water
in and out of lakes and nature created its own way to keeps lakes under control
and clean so that all inhabitants could live a normal life. To allow us to enjoy
beautiful relaxing ponds full of lively happy and colourful fishes we invented
the biofilter. BIOFILTERS are essential in almost EVERY fish pond.
What Exactly Is A Biofilter?
At their simplest biofilters are literally black or green boxes square or
round, fat and thin or whatever. Often they hold sponges, brushes or other means
of trapping solid particles. The boxes also contain plastic tubing often called
Flocor, plastic balls, other weird plastic shapes, lava rock, special tape,
string bags and coke bottle tops. If you are lucky the boxes or containers contain Alfagrog
just like the FishMate gravity and pressurized filter products stocked by
The collective name for these objects put into the filters is biomedia ie a medium (better still
surface) on which biological activity can take place.
The sole specific purpose of biomedia is to create surfaces on which
specific bacteria can live in order to convert the waste products naturally and
continuously produced in the pond to less polluting materials. The sponges and
brushes are specifically there to play a role of removing some of the solids
that get circulated via the pump.
A biofilter first and foremost is designed to purify water and the secondary
purpose is to remove the solids that make water cloudy. Few people starting off
with a pond realise this.
For any biofilter to work well the following is needed:
Reasonably good mechanical filtration (solids removal) before the water enters
the biological chamber in the biofilter. The better the removal of solids the
better the biofilter will work.
A large colony of specific bacteria that literally eat the waste products from
the fish. The bacteria convert this waste to nitrate chemicals, which act as a
kind of fertiliser for plants growing in the pond. The larger the colony of
bacteria the more quickly they can destroy waste build-up.
A continuous supply of oxygen to enable the bacteria in the filter to live.
This is why a pump must work 24 hours per day since the water is the source of
oxygen for the bacteria and without the oxygen the bacteria die quickly and
certainly within 8 hours. Once dead the colony takes 4 to 6 weeks to build up to
a reasonable working level again.
Good water flow rate throughout the filter biomedia. And not just through
portions of the bed easier said than done by the way so biofilters tend to be
over-designed to compensate.
For effective total filtration of a pond the mechanical aspect of solids removal
also have to be good.
Good Pond Bio-Filter Design Principles
There are more advanced methods (like the new Hozelock Trinamic pond filter) but
a simplified way of viewing factors
affecting good filtration are discussed here.
A filter will do a good job if there is a way of removing solids effectively and of
colonising large surface areas of bacteria that are able to intimately contact
the water containing the waste products pumped through the system and if there
is continuous flow of oxygenated water through the biofilter, tend to use
the word filter as a short version of biofilter.
The more surface area that can be incorporated into a filter the better. An
attempt (albeit a basic attempt) to achieve this is by placing hollow tubes of
rough plastic inside the filter box. Other attempts include the use of orange
bags, lava rock, tubes, and even hair curlers.
Certainly all of these methods create holding areas for the bacteria but they
are all inefficient and therefore relatively large sized boxes are required to create
sufficient area for the bacteria colonies to grow on. The generic term for all
these substrates used for holding bacteria is BIOMEDIA - a medium to hold
Foam sheets by virtue of the many small holes in them also provide surface on
which bacteria can grow as well as acting as a mechanical filter to remove
Alfagrog and FishMate Pond Filters
By far the best means of getting large surface area into a small black box
biofilter at low cost is to use porous ceramic materials like Alfagrog. This
is a product made in the UK specifically for fishpond water purification. It has
a massive surface area of between 40 and 100 sq. metres per litre depending upon
the particle size compared to 0.2 square metres per litre for plastic tubes.
Without getting too technical this means that 1 litre of Alfagrog can hold the
same amount of bacteria as 200 litres of plastic tubes. In other words a box
using plastic tubes needs to be 200 times bigger in volume than one containing
Alfagrog to get the same biological performance.
Alfagrog looks a bit like cinders, it is lightweight, comes in different sizes
and you can literally blow through it because it is so porous. See the image.
Read more about the fascinating subject of pond
filters and biofiltration