For the umpteenth time in my life I heard the FACT that a filter had to be 1/3 the pond size whatever that actually means. The other party was not impressed by my views and once my talk started there was one less listener. Let me try hard to debunk this myth of biofilter size and explain what is really really important. If there is a need to talk about size at all then this must relate NOT to pond volume, NOT to filter box size, and NOT to relationships between pond volume and filter volume. Size in terms of effective biofiltration ALWAYS means size (rather speak of quantity) of surface area where the bacteria, oxygen and food source come into contact with each .... ie at the biomedia surface.

Of course we all talk of a biofilters being suitable for a pond up to x gallons because we have to try and simplify the understanding for both sellers and buyers of off-the-shelf products. And of course we all generalise in order to communicate "better". I
have just invented a new product called Biostraws .... be patient There is a product called Flocor which is sold around the world as a biofilter medium ... a biomedium is a place where bacteria can happily live by clinging to a surface that comes into intimate contact with water containing nutrients and oxygen. Flocor is one of thousands of possible media.

They have just done a great job of marketing their product over the years. Just in case someone does not know what Flocor, let me describe it ... it is a hollow black plastic tube with a few ridges. It is about 1 inch diameter and 2 inches high, normally black. The ridges by the way are cosmetic to all intents and purposes.

My Biostraw Invention

Now ask yourself a question? What is the difference between a Flocor tube and a drinking straw of the same height. Then ask your self another question ... how many pieces of drinking straw can be fitted into the empty space of a Flocor tube?

The answer is quite a few. Let me put this another way. We have all seen the boxes of drinking straws which say 100 for $1.50. Sure enough just like the box says there are 100 tightly packed straws inside this rather small box. Imagine if you took all the tightly packed straws out of this small box and then filled the box with Flocor tubes in a random fashion. Count the number of Tubes you can fit into the box. Now calculate the areas of available surface for the two systems. To save you the effort I have done it for you.... You will find that the surface area of the drinking straws that fill the box (ie all 100 added together) is about 6 times that of the Flocor tubes that also fill the box. Put another way a drinking straw is 6 times more efficient as a biofilter medium than Flocor is. In turn this means that theoretically a box biofilter containing drinking straws can be 6 times smaller than a box biofilter containing Flocor because the amount of space the bacteria require is now reduced by 5/6 .... I can here you say; amazing, isn't it.

It is amazing; yet remarkably simple when you think about it. You may use my invention if you wish ... no charge. This basic fundamental understanding is the very essence of good biofilter design. Bacteria are tiny and do not need lots of fresh air space inside any box. They just need somewhere to attach themselves ... by the way every surface inside your pond: the walls, the stones, the pump and its pipes are also biofilters. The bacteria live on these surfaces too. In practice
it does mean that adding drinking straws to a biofilter full of hair curlers, plastic balls, plastic tubes, and modern plastic wonder-shapes claimed by the biofilter sellers to be better than anything since sliced bread will increase a filters biological capacity significantly. Let me come back to the old story of a biofilter must be 1/3 the pond size.

This goes back to the earlier days of pond keeping when biofilters were actually large chambers filled with stones or gravel and water was distributed across the surface of the stones. As water worked its way down the bed of stones the bacteria would convert the ammonia waste to nitrates so long as sufficient oxygen was present. However in deep beds of gravel there was a danger of anaerobic bacteria problems developing bcause channelling would occur. Frequently beds were
too deep so that all the oxygen was consumed before the water reached the bottom of the bed to be re-circulated.

This is where the 1/3 rule of thumb came from and it was based upon area of the bed not volume. Understand what is written above and you can examine all sorts of ways to improve your biofilter and/or reduce the size of it and of course it becomes really easy to make one for yourself. You have heard me mention Alfagrog many times ... Alfagrog is 40 times more efficient than Flocor. You will come across the term SSA which means Specific Surface Area. It is a means of comparing different filter media. The units used are SURFACE AREA PER UNIT OF VOLUME. For example 2 sq feet per gallon or 5 sq metres per litres etc. Flocor's SSA is less than 1 sq metre per litre while Alfagrog is 40 sq metres per litre.

Surface area required in total for bacteria is dependant upon many things but especially the amount of food fed to your fish. Since larger ponds tend to have more fish which need more food then such ponds need more biomedia. This is why we simplify biofilter selection by talking about pond volume. Now you know better ... much better.

Knowledge is power when we talk about pondkeeping.