In message , The Wanderer
writes
Many garden centres do a 'clear feature' additive for water features.
Bleach would do as a temporary measure, but it degrades in a matter of
hours in strong sunlight. Only add a very small amount, as it might
harm any rubber pipework, couplings etc. That presumes, of course, that
it is just a water feature and doesn't have any fish.

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.

--
Clive Mitchell
http://www.bigclive.com