Ah, Malapterurus Electricus! One of my favorite species!
E-cats will kill just about any tankmate with a few exceptions:
1) Fish with scales is a bit more resistant to electric shock
2) Scaleless fish is much more vunerable (except synodontis cats)
3) The larger the tankmate is, the less it will be affected by e-cat's discharges
4) Don't try to keep these with super aggresive cichlids, etc. E-cats are easily damaged, as their skin is quite soft. Also, this fish "charges up" -- stores up electricity over period of time (few hours), and then discharges in a series of bursts. The first burst is the strongest, and they get gradually weaker until the charge is depleted. I once tried to house an 8" electric cat with a 10" tilapia butticoferi thinking that the e-cat can keep the beast at bay. Butti quickly learned that after a painful initial shock, e-cat becomes less dangerous and explored this weakness for a couple of days. I ended up separating them in order to save the e-cat
5) I have successfuly kept e-cats on 2 separate occasions with various species of synodontis (eupterusm, decorus, ocilefer, and "lace catfish"). Not sure why but synos are not affected by e-cats electricity at all
6) I have also succeeded in temporarily housing a 6" e-cat with equallu sized bichirs -- never observed any ill effects
7) do not attempt to keep 2 e-cats together -- they will get along just fine as babies but will definetely begin "ripping each other apart" when they get older. Here is a series of shots depicting a pair of 8" electic catfishes that coexisted peacefully for a few months and just went berzerk one morning (I am almost certain that if not promptly separated, they would have ended up killing each other):
