I have never known one to jump. They are usually pretty mellow. Must be something going on. As renogaw mentioned, check for stray voltage. I just hooked up a brand new, never used Quiet One 4000 for sump return and when I was putting my hand in the tank I was feeling a tingleing in a small cut on my finger. I un plugged everything one at a time and found the 4000 to be giving off stray voltage. I plugged in the ground probe and it went away. I can imagine what the livestock must have been going through if I hadn't discovered it and decided to use the ground probe.
They do work and even if this is not what is going on with your mandarins, everyone should use them.
Sorry about your fish, hope you find out what it is.