Pond filters and biomedia

I get more queries associated with biofilters than anything else.

This is not surprising since a biofilter is so important in any fish pond .... A biofilter for a fish pond is essential, it is not just a nice-to-have item.

Selection is much simpler than you think especially for small to medium sized garden fish ponds. The following points are important even if you build your own biofilter.

Specification is based upon pond volume with consideration given to quantity of fish if you really overstock the pond.

The size of the biofilter box or container is almost totally irrelevant .... many people will tell you that the filter should be a certain percent of the pond volume. This is quite simply WRONG.

Such advice will cost you more money than necessary and will not help you.

The biomedium used and the flow rate through the biofilter are important to determine the size of the box or container. As far as flow is concerned see the end of this article since many people misunderstand and believe all water must flow through the filter at the same time.

In simple terms the biofilter contains media on which a large amount of specific bacteria grow. These bacteria are required in order to digest the pollutants created by fish bodily wastes building up in the pond water.

Think of the bacteria as clinging to all possible surfaces in the biofilter box. This means the more surface there is then the greater the space for the bacteria to cling to and the more the amount of bacteria there can be ... simple as that.

The more bacteria you have the quicker they convert fish waste to harmless nitrates and the better your water quality will be.

Ideal surfaces come from media with rough porous surfaces (not smooth plastic or stones or very small packed particles like sand. It is the combination of roughness and porosity (more important than just roughness) which creates large areas of surface for bacteria colonisation.

We talk of SSA ... specific surface area in a fixed volume of media... Look for biofilters which contain biomedia with high SSA.

To really push this home take the situation of two different biomedia

The first has an SSA of 50 sq feet per gallon, the second an SSA of 100 sq feet per gallon.

The box for the first medium will have to be twice as large as the second one in order to hold the amount of media that will do the job of biofiltration adequately.

Many commercial units are very overpriced and no better than a home made one. From what readers tell me Rubbermaid make some great low cost containers you can use.

By the way it appears Alfagrog is NOT available in USA. It is an UK product. We have it in South Africa ... I have been putting it into all my biofilters for years.

NOTE IMPORTANT.... If you are pumping more water around your circuit than stipulated as the maximum flow for any biofilter then all you have to do is split the flow so some goes into the biofilter and the rest by-passes

the actual filter box and goes to your waterfall for example.

All the pond water eventually goes through the biofilter. In fact this will happen about 12 times a day for most ponds.

The more serious koi keeper would prefer that all the water is passed through the filer 24 times per day.

If you want to calculate how many times your water is flowing through the filter every day just divide the flow through the filter into the ponds volume. For example if you are pumping 500 galls per hour and the pond is 2000 gallons then all the pond water if filtered every 4 hours which is the same as 6 times per day