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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 Bradshaws.. 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:

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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 bacteria.

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 solids.

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 ceramic biomedia

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


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