I would definitely start with getting a test kit and checking out your water. There are lots of sites online with information about 'Ick' and it's treatment, most are for the aquarium but hold true for ponds for the most part but in a larger scale.(You just don't have the control in a pond that you do in an aquarium). Like Mark says, you may want to get a Pro in to check things out if there are no obvious reasons for the spread of the disease or it's rapid occurence.

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