copper isnt a very powerful conductor, and is poisionous to fish, particularly scaleless fish, I wouldnt use it. It would be important to use a metal that is not soluble in water, so it does not affect the conductivity or chemistry at all. I believe that electrogenic fish create an electrical (and thus magnetic) field around themselves, that would move the electrons in waves which are both sent and recieved by the organ, with varying intensity and frequency being stereotyped in ways used for communication, gender identification, and location purposes. Thats a really vague way to say it, but it leads to the conclusion that you would not nessecarially have just spikes in voltage between the wires in the tank that would inherently make noise (i dont know how much voltage it takes to make a speaker pop, but that would depend on the size/kind of speaker), but instead you might just have a fluctuation in the conductivity sent as electrical waves through space. You could find out the nature of the signal with a miliammeter, if you put the electrodes in the tank, to find out if it is a quick voltage spike, or if it is a fluctuation, you might be able to see it on an oscilloscope if you had electrodes in the water. But I'm not confident that the nature of the electric field produced by a knifefish would inherently make a sound on a speaker, you might get pops if its a quick spike in voltage, but I wouldnt know how that translates. I saw a video on youtube of somebody doing this with an elephantnose, but i think they were also doing it with a piezo buzzer or something similar, just a small transducer that might pop and crackle a little bit when the volts went up and stuff. Because thats what it did.
