Waterproofing a wire for fish tank
You can apply "aquarium grade" silicone directly to the wire and (after curing fully) it will work, BUT: If your fish already ate insulation from one section of your wire, maybe the better question here would be: How do I keep my fish from eating more insulation off of my filter wire(s)?" 
Any fix you use can only be as good as your ability to keep the fish from simply chewing off the original insulation from another spot. A possible solution to that may be to slit a piece of aquarium air tubing lengthwise; then push the vulerable part(s) of the wire (after repairing current damage) through the slit so your wire's insulation is now "armored" by being wrapped in the harder-to-chew air tubing.