There is brush on electrical tape that would work... but i'm not sure how fish safe it is :S
Here's a decent writeup on it:
http://www.epinions.com/review/BlueM...614307972?sb=1
One thing you'll have to find is something that has some sort of UV protection. Even electrical tape will turn to a solid cracking mess in time under your lights. Unless of course you have LED's, then I'm not sure how much it would fray. I had some wires in my canopy that where exposed to my halides that made the electrical tape toast.