The major purpose of a biological pond filter, in any fish pond is to remove ammonia, which can be highly toxic to pond fish and other aquatic pond life. The biological process responsible for removing ammonia is known as the Nitrification process and is extremely important if you want to maintain a healthy ecosystem, suitable for aquatic life to thrive in. The secondary function of a biofilter is to mechanically separate leaves and other organic matter from the pond water. I have written this article in the hope that it will help to highlight the failings in my opinion of using a mechanical sand filter in any garden pond.
Why Are Sand Filters Bad For A Garden Pond?
Before I discuss my reasons why I believe that this filter system has no place in a garden pond, I would quickly like to add that I am referring to a mechanical sand filter and not a fluidized sand filter, which has been designed for pond use. Ammonia is present in all fish ponds where there is biological activity such as fish feeding, rotting vegetation and decomposing aquatic life. It is important to remove this toxic chemical as soon as possible.
In order for the conversion of ammonia into nitrite and then into nitrate to occur, certain criteria need to be met: There needs to be a plentiful supply of oxygen flowing through the pond bio media; a large colony of Nitrosomonas and Nitrobacter beneficial bacteria need to present and there has to be turbulence, which mixes together water, oxygen and ammonia to speed up the breakdown of ammonia.
Disadvantages of A Mechanical Sand Filter:
* It only has a low surface area on which nitrifying bacteria can colonize.
* This type of filtration system encourages the build up of unwanted heterotrophic bacteria that is bad for pond water quality, unlike nitrifying bacteria that is necessary. If you allow heterotrophic bacteria to develop in your pond you will see some nasty results, including the production of hydrogen sulphide (rotten egg smell), ammonia and carbon dioxide. Let's think about that! We are using a sand filter to remove ammonia but it actually produces ammonia.
* A sand filter is costly to operate as it needs to be back flushed between 5 to 6 times per day for at least 5 minutes each time. This requires a lot of water, which means that if you are on a meter it can be quite costly.
* As water flows through the sand bio media, channelling starts to occur. Channelling (holes in the sand) allows water to flow straight through the sand without being filtered. This requires even more back washing.
What Type Of Biological Pond Filter Should I Use?
As far as I am concerned the only viable solution for effective pond filtration is to use a biofilter or a veggie filter, although a veggie filter can be high maintenance. If you have a small to medium sized fish pond then a pressurized biological filter is a really good choice, for the following reasons:
* It is compact and is easily hidden from view, preventing it from being a bit of an eyesore. Traditional black box filters are larger and are harder to disguise.
* Pressurized bio filters have an inbuilt back flushing system which makes it easy to purge unwanted organic waste such as trapped leaves and grass. This makes routine cleaning easier.
* The pressurized operation means that this filter type can be located away from the pond edge and not at the highest point of the pond, unlike a box type filter which relies on gravity. My recommendation would be to use either the Oase Filtoclear, Hozelock Bioforce or the Fishmate Powerclenz, mainly due to their cost, reliability and excellent warranties.
For large Koi ponds the best form of biological pond filtration is a bead filter or a vortex Koi filter. Bead filters contain up to 600,000 beads per cubic foot. This means that there is an extremely high surface area on which beneficial nitrifying bacteria can colonize. These beads are suspended in a turbulent chamber of water that is constantly full of oxygen and ammonia. This provides optimum conditions for ammonia to be broken down. A vortex filter is the preferred choice for many professional Koi keepers as it is low maintenance and has been tried and tested over the years.
The 2 equations below show the quantity of oxygen needed by nitrifying bacteria to oxidize ammonia (oxygen is represented by O2). As you can see it is needed in abundance.
* For Nitrosomonas: 55NH4+ + 76O2 + 109HCO3- ---> C5H7O2N + 54NO2- + 57H2O + 104H2CO3
* For Nitrobacter: 400NO2- + NH4+ + 4H2CO3 + HCO3- + 195O2 ---> C5H7O2N + 3H2O + 400NO3-
To conclude this article I would seriously advise you not to install a mechanical sand filter in your garden pond. If you currently use this type of filter, remove it and replace it with a biological filter that will purify the water and not pollute it.
