Another way of killing the little blighters is by pouring a bit of oil on the top of the water, if it's only a bucket though i'd do what woofgang says and pour it all away.
Oh I never knew that. I've got a new pond/waterfall and it's full of the little blighters. But every day I get loads of sparrows and tits coming to drink and play there, so I don't want to poison them as well. Is there anything I can put in to kill the larvae but not harm the birds?