Bradley Walsh: Egypt’s Cosmic...
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For more on marking an answer as the "Best Answer", please visit our FAQ.You get a certified electrician to take a new spur out of the house and run armoured cable to your pond. You run a pump to circulate the water through a filter, preferably with a UV tube
http://www.rhs.org.uk/advice/profiles0404/pond_filters.asp
It is expensive but it's the only real solution.
You can make it a lot better by shading the pond, direct sunlight makes it much worse, making sure the pond is large enough for the fish and using certain plants.