I don't expect it's possible to observe magnet deflection from an eel-powered circuit. The burst is just too short: by the time there's been enough force to turn the magnet aside, the voltage drops back down to nothing and friction leaves the magnet in place.
If your experimental setup is to have a wire run from the exotic fish, around an iron bar several times, and back to the other end of the fish, with a secondary wire also wound around the iron bar and across a spark gap, I actually don't know what you'd see if you agitated the fish enough to get it to shock as fast as it could. It's quite a weird experiment to do, though.