Koi

Ben Helm says...

The best time to service or clean a filter is when the Koi are at their least active in the pond, and therefore least dependent on the filter's funtion – early spring or late autumn. Unfortunately, this is also when the filter is less likely to need regular maintenance. When Koi start to feed more actively you will also have to clean out the mechanical chambers (or media in a pump-fed filter) more frequently. For a pump-fed filter, where the media usually doubles as both mechanical and biological and is exposed to the relatively oxygen-rich air, this is not a problem as long as it is cleaned in pond water. In a gravity-fed filter, mechanical chambers can be purged independently of the precious media in the biochambers, enabling you to preserve your filter's maturity. In the warmer months when bacteria are more active, and when the water holds less oxygen, this should be carried out as quickly as possible. The addition of airstones in the biochambers will reduce any adverse impact this short period of downtime may have on your filter's performance.

Ben used to farm Koi for a living and has published three books on ponds. See his website watergardenersbible.co.uk for more details.