A good pond filter system is required to keep a pond clear so that it provides a clean and healthy environment for pond life such as frogs and fish to thrive. Filters are necessary to protect the pump from debris, to break down fish waste and other organic waste, to remove particles and reduce algae growth. Good pond filters use a combination of biological, chemical and mechanical filtration to achieve these functions. Biological filtration uses bacteria to break down organic waste and mechanical filtration traps particles for removal. Filters do not keep the water completely clear and the smaller the filter, the smaller the volume of water it can clean.
DIY pond filters can be bought from garden and pond suppliers, some of which now sell their goods through the Internet making shopping much easier. There are many types of DIY pond filters:
External filters are a popular choice of DIY pond filter because they are easiest to install and service as they are out of the pond. However their size makes them hard to hide and they can look unsightly. Filters may be used in combination, for instance intake filters are not a suitable choice on their own as they do not improve the water quality but can be used in conjunction with other filters. Another disadvantage of intake filters is that they need cleaning daily whereas the other types can be left days to weeks.
Some pond filters must be installed at the highest point where water exits but pressurised pond filters such as inline filters can be installed at any level. Choosing the correct size is important - the larger the filter, the more effective it is. While a small filter is better than none, an under-sized filter can restrict the flow of water through the pump, causing leaks in the system. Generally goldfish ponds should be filtered at a rate of one half of the pond's volume per hour. Another consideration is that ponds which are in direct sunlight for most of the day will require a larger filter.