Fish Pond Bacteria, Biomedia and Pond Filter Size
Fish pond bacteria grow on the surfaces of biomedia encased in a pond filter as well as all other surfaces in a fish pond. It is important to understand surface area in terms of biomedia choice. You will save lots of money on your biofilter this way and making your own pond filter becomes easier and cheaper.
The more bacteria you have in any pond filter the better your pond water quality will be. These filter bacteria transform toxic chemicals in the pond water secreted by the fish into plant fertilizer suitable for pond plants. Choosing the correct filter biomedia has a major impact upon your pond filter's performance
I get so many queries about biofiltration and since it is so important to any fish pond I am going to bore you (hopefully not) with a bit of basic maths.
If you take a plastic tube, cut it open and flatten it then you can measure or calculate the area exposed by the surfaces (front and back). For simplicity's sake we will assume that the thickness of the tube was very thin and ignore this bit of area. In other words we will just have two sides on which the bacteria can live. The total area is therefore as follows
Length x width x 2
However you can calculate it by using the formula below without cutting the tube .
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Area for bacteria to colonise = 2 x 22/7 x diameter of tube x height of tube
22/7 is the famous pi as in pi r squared. Just remember you mathematicians this is a very thin tube.
Lets take a tube 2 inches high and 1 inch diameter typical of many filter biomedia.
Area = 2 x 22/7 x 1/12 x 2/12 = 0.1 square feet
So each tube has an area for filter bacteria to colonise of 0.1 sq feet.
Lets assume that our fish pond theoretically needs 1,000 million bacteria and 0.1 sq feet can provide space for 1 million bacteria.
Then to get effective biofiltration our biofilter will need 1,000 tubes inside it.
It just so happens we can also calculate the volume of 1 tube and it is 0.0012 cubic feet approx.
So 1,000 tubes would occupy a total volume of 1.2 cubic feet if they packed perfectly which of course they do not. In practice a container of about 2.5 cubic feet would be needed to hold the 1,000 tubes
Now if you doubled the weight of fish in your pond then you would need 2,000 tubes and the box would need to be double in size also ... 5 cubic feet. Yes the box is starting to get pretty big and 2,000 tubes cost a lot of money even if they are only black plastic.
Now just imagine if you could find a magic kind of biomedia that was the same physical size as the one above but had 40 times more area. Suddenly you would only need the equivalent of 25 tubes and not 1,000. It would also mean that the filter box could be 40 times smaller to hold the same amount of surface for the 1,000 million filter bacteria to live on.
The point of this exercise was to show that a biofilter box size depends upon the type of biomedia used it does NOT depend upon the size of the pond.
Now you know why a statement such as a biofilter MUST be 1/3rd the size of the pond is illogical and frankly nonsense.
The other fundamental lesson from the exercise above is that any biofilters performance can be improved by adding more efficient biomedia in place of or in addition to the existing low efficiency biomedia in the existing filter box.
Imagine for a moment what a magic tube might look like. Well think of our tube above and then cut up thin drinking straw into lots of 2 inch pieces. See how many you could fit into the 1 inch diameter tube . Quite a lot. By doing this you have created far more surface onto which bacteria can live without changing the space (volume) or size of a box to hold the biomedia.
Remember these filter bacteria are tiny tiny creatures that live in the smallest of spaces.
Yes you have guessed it . Drinking straws would make an excellent biomedia. Save them all up from now on and put them in the biofilter just make sure they do not float away.
As a matter of interest a box filled with drinking straws would have to be about 16% the size of a box containing 1 inch diameter tubes of the same length in order to have the same biological capacity. In other words you could squeeze 1,000 million bacteria into a filter box 1/6th the size.
Put another way drinking straws are 6 times more efficient than 1 inch diameter plastic tubes of the same length.
There are more convenient magic tubes as you will see later but hopefully you have a full grasp of how to improve your biofilter by now.
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 fish pond 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. Learn how a pond filter works.
"The Complete Pond Solver" by Tony Roocroft
2005 Edition. Essential reading for anybody who wants or has a garden pond. Get 12 Excel pond calculators free as well as "Water Lilies and Pond Aquatics" ebook also free
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