"Clive Mitchell" wrote in message
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If you wanted to go really technical you could add a UV steriliser in line
with the water flow, or even an ozone bubbler that effectively acts as a
continuous biodegradable bleaching effect. These are all things that
could be sourced on ebay for Koi pond use.

That would be my choice, if I didn't just leave the pond pump running
throughout the winter. It only gets switched off if it seems likely that the
pond will freeze over and the Medway is not so much further North that it is
likely to be a lot colder.

Colin Bignell