Biological filtration for koi ponds, fish ponds and aquaria- why do we use filters?
Let’s be clear from the start (excuse the pun), the primary reason for having a biological filtration system is not to provide clear water – although that might be a beneficial side effect. The purpose of using a filter is to improve water quality by removing various pollutants, such as ammonia, nitrite and solid organic waste, which would adversely affect fish health if they were allowed to accumulate.

Are filters important for fish health and are they more important for some fish than others? 
The filtration needs of most aquarists are modest – indeed the well-planted ornamental goldfish pond is usually self-purifying.  At the other extreme most koi keepers are often heavily reliant on biological filtration to maintain good water quality. With high stocking densities of large messy fish in often-barren ponds the average koi pond is usually a very unnatural environment. The addition of a bio-filter extends the minimal self-purification capabilities of the pond by removing toxic compounds such as ammonia and nitrite and thus allowing a relatively large number of fish to be kept in a relatively small volume of water. The implications for fish health if the filter doesn’t function properly are obvious! Indeed, it is my experience that misunderstandings about filtration and poor filter maintenance are very often the underlying cause of many koi health problems.

Does removing ammonia and nitrite mean good water quality?
Yes and no. Maintaining very low levels of ammonia and nitrite will improve water quality, but these are not the only pollutants, especially in a heavily stocked tank or koi pond.  A simplistic view of filtration focuses on simple nitrification – that is the conversion of metabolic ammonia to less toxic compounds.  However, any chemical or substance present at higher than normal levels, even if it is not directly toxic, should be considered a potential pollutant. So therefore any filtration system, together with routine  maintenance should be designed to remove not only toxic products such as ammonia and nitrite, but also other organic and inorganic wastes products.

So what is the ideal filtration system? 
A simple question without a simple answer. This will vary according to what is required; be it a small marine tank, a well-planted ornamental garden pond or a 10,000 gallon specialist koi pond. However, by understanding more about the how a filter works, the loading put on it and the type of maintenance required we can arrive at some basic guidelines and then you pays your money and takes your (informed) choice!  The rest of the filtration pages cover the various aspects in more detail.