Some ponds have muskrats that live around the pond, and some times they can play havoc with electrical cords. Floating pond fountains and surface aeration can be affected by them. They seem to only bother chewing them at the fountain and about 6-10′ from it under water.
Here we have a picture of a wrap that we use in these cases. It can be put over the cord or just wrapped around to stop them from chewing the cords. We can easily put this on existing fountains or if you have muskrats and are buying a new fountain ask for it to be installed on the electrical wires.