Bacteria grow on the surfaces of biomedia ...

it is important to understand surface area in terms of biomedia choice. You will save lots of money on your biofilter this way. The more bacteria you have in any pond filter the better your pond water quality will be. These bacteria transform toxic chemicals in the pond water secreted by the fish into plant fertilizer suitable for pond plants. Choosing the correct 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 simplicitys 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 ..

  • Area for bacteria to colonize = 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
  • Area = 2 x 22/7 x 1/12 x 2/12 = 0.1 square feet
  • So each tube has an area for bacteria to colonize of 0.1 sq feet.
  • Lets assume that our 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.

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 25 tubes and not 1,000. It would also mean that the box could be 40 times smaller to hold the same amount of surface for the 1,000 million bacteria to live on.

Well you can find such magic tubes. They do exist, as you will soon see.

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 I get into trouble when I argue with people who say a biofilter MUST be 1/3rd the size of the pond. It is illogical and frankly nonsense to believe this. Hopefully you agree.

The other fundamental lesson from the exercise above is that any biofilters performance can be improved by adding more efficient biomedia (some magic tubes) in place of or in addition to the existing low efficiency biomedia in the existing 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 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 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.

Good Pond Filter Design Principles

box bio filter with brushes A filter will do a good job if there is a way 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 we 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.

Alfagrog is called Supra in USA Alfagrog is called Supra in USA and can be bought here ... it is best all purpose media for small to medium ponds . 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.

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