There have been many frequent questions asked regarding Koi filters as to why some Koi ponds still remain a pea soup colour even with a biological filter installed. It seems that everything is in place yet the desired result of clear, clean healthy water is not being obtained. There are a few factors to observe here that could lead to such an undesirable affect. But before we delve into that issue, I want to point out that even though the most talked about process of biological filtration is the converting of ammonia into nitrate, there is much more going on that should be made aware.

Within Koi filters, and also within the pond water, there are a wide array of biological, metabolic and chemical activity taking place, each of which is producing it's own range of pollutants. This is proceeding as organic waste, predominantly fish faeces, decomposes. The longer this waste is left to decompose, either in the pond water or in the filter, the more these harmful processes are occurring. Without regular cleaning, of both the pond water and the filter, production of numerous DOC (dissolved organic carbon) compounds are left to build steadily, increasing the level of pollutants in the water and in the filter thus increasing, and possible defeating the work of Koi filters in general.

One point to take note of is the size of your pond in relation to the size of your filter. To be more exact, the size of surface area within the filter itself. It's understood that most types of pond filters these days rely on bacterial growth within the filter media we provide in our pond filter chambers. The nitrifying bacteria attach themselves to the filter media producing a slime layer of biofilm. This biofilm contains microorganisms, which feed from the water that passes over them within the filtration system. Now here is where it really gets interesting. In order for these bacteria to function and feed properly, there has be a sufficient amount of surface area for them to grow plentiful. The media also has to accommodate enough void size, or empty space, for the water to flow over them slower and longer, providing the microorganisms within the biofilm as much time as possible to take in the harmful pollutants contained in the water. We achieve this through Koi filters by using media with adequate SSA (specific surface area) matched with sufficient retention time.

If the right combinations of overall filter size, SSA and retention time are not met, then the breakdown of DOC compounds into simpler inorganic compounds also might not be sufficient. If this is the case, then eventually these compounds are incorporated back into living organisms. This insufficient filtering process in turn pumps these intermediate products and organisms back into the pond water, thus supplying algae with endless amounts of nutrients to thrive on.

There are numerous varieties of media available for Koi filters, each yielding different results in terms of SSA, for potential biofilm growth, and retention time, the total time water spends flowing over the biofilm. These include foam, gravel, sand, plastic and matting just to name a few. In regards to algae, a pond filter which incorporates UV filtering is preferable as this kills algae in the water passing through the filter. However, keep in mind that this process of filtration only works on green water algae. Blanket weed algae is a different kettle of fish altogether, excuse the pun, and requires other means of eradication as it grows along the sides of the pond and therefore does not pass through Koi filters.

If you are experiencing these problems, then it would be a good idea to speak with your local pet store or pond specialist to discuss if your filter is sufficient, and if not what can be done about it. It could be a simple matter of either changing the filter media you are using, cleaning your pond and filter on a more regular basis or perhaps even installing a second or larger filter.