Learn All About Fish Pond Filters & Pond Water Quality Part 4 of 4

Biofilters summary

To summarize ... biofiltration capacity in any system can be enormously increased by replacing traditional plastic tubes by more efficient biomedia. Alfagrog is such a biomedia . It is a true magic tube. Alfagrog is known as Supra in USA and can be bought here

Alfagrog fantastic low cost biomedia ...

Why? because it is extremely porous, full of tiny tubes if you like into which bacteria find their way and do their job. To keep the Alfagrog efficient it is best to keep as much solids as possible away from the surface. This is why cleaning the Alfagrog in a bucket of pond water now and again is recommended (make it easy by placing Alfagrog in string bag)

 If you suspect that your filter is not working well enough then add a few litres of Alfagrog for a big improvement in biological and probably mechanical filtration. A single litre of Alfagrog will cater for a pond of 1,000 litres (about 240 US gallons) from a biological point of view.

The challenge in creating good effective bio filters is to get as much surface area inside your black box as you can. A secondary benefit of using Alfagrog is that the black box can be made smaller making it easier to install in the pond environment.

The important point to remember for the garden pond keeper is that it is NOT the size of the box that determines the capacity of a biofilter. The capacity is determined by the amount of biomedia surface area in the box.

In my opinion and also by independent testing the best run-of-the-mill garden pond filters contain Alfagrog. The best koi pond filters contain Japanese matting inside vortex filters for sound technical reasons to be discussed later.

The following points are important to consider when choosing a basic black box biofilter.

Good solids removal capability as evidenced in some pond filters by the use of open cell foam that fits into the chamber or box without leaving gaps around the edges.

Ideally there should be at least two layers of foams with different sized cells making the removal of solids progressively more efficient. The coarse foam should always be on top.

Water should ideally be distributed evenly onto the foam surfaces to get better distribution of water across the surface of the foams . Not easy to achieve in practice.

The picture below gives a good idea of what a typical filter might look like when you open the lid and peer inside - note the different foam layers and the spray bar as well as the overflow pipe. The spray bar is probably a nuisance rather than a help and it will restrict flow of water into the filter. Take it out by all means.

Biofilters, the normal black box type

The box type filters with loose fitting lids are all gravity devices. In other words the water is pumped into the box but it flows out under gravity and not pressure. Consequently the filter is normally at the highest point in the circuit - i.e. above the waterfall.

If you have a waterfall and it is not convenient to install the filter above the waterfall then distribute the water from the pump so that part goes to the filter that is at pond level and part goes to the waterfall using a separate pipe.

You will need a 2 way valve arrangement like the one below or at least a Y piece. In this way the water from the filter can go straight back into the pond - obviously the outlet from the filter must be slightly above the water surface in the pond. This mechanism will normally require a slightly larger pump or better still install a separate small pump to run the biofilter on its own circuit as discussed earlier.

How to select a pond biofilter ... the basics

To specify an appropriate biofilter it is important to know the volume of water in a pond to say about 80% accuracy. A filter is specified in terms of what capacity it can handle when the pond is stocked normally which needs some interpretation

Typically therefore a filter will claim to handle or purify up to 5,000 litres of pond water. The claim will be based upon qualifications such as fish stock density, amount of food fed and so on. If you can afford it always go slightly bigger to get better performance.

Installing a biofilter is no guarantee of clear water (although it is possible you will be told it is do not believe it) because clear water is dependent not just upon the filter design. It is dependant upon other system components like churning because too large a water flow is passing through the pond. Another reason for cloudy water is the presence of algae, which are far too small to be taken out by all the pond filters on the market that I am aware of. However a well-designed filter will go a long way to giving you clear water (where the lack of clarity is resulting from solid particles and not algae).

If you intend to keep fish you must have a biofilter unless you want impure water and dying fish. I will discuss natural biofilters later.

Do reconsider your intention to build a pond if you are not prepared to spend money on a biofilter. You will be wasting money if you go ahead without a biofilter and I promise you will do it sooner or later or you will turn the pond into a sand pit.

"The Complete Pond Solver" by Tony Roocroft  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. THis page is an extract form this book