I haven't done a count, but suspect perhaps half of the many aquaria I've visited with descent-sized tropical freshwater exhibits have a mormyrid. Zootierliste is generally good, but unreliable when it comes to smaller fish species; there are quite a few European aquaria with mormyrids not listed there. By far the most common species (public and private aquaria alike) is Gnathonemus petersi.
However, in "generalist" aquaria they're often hard to see, as they tend to be light sensitive. E.g. a few places I've seen "Congo River" aquaria with all the usual species, as well as a mormyrid; often not visible under those circumstances. (I should add that the few Campylomormyrus species I've seen have always been kept in aquaria with little current; members of this genus generally originate from places with fast current/rapids and may behave differently if kept in an aquarium simulating that.)
Kept in dimly lit aquaria, as done a few places, they're quite active, interesting and easy to see. The set-up described in an earlier post from Taronga Zoo is also done a few other places; most recently I've seen it at Blue Planet aquarium in Copenhagen with three species of mormyrids and African knife fish (though some may miss that, as only one of the four species had a sign on my visit a few months ago).
BTW: Gymnarchus niloticus isn't a mormyrid. While there are some behavioral similarities between this species and mormyrids, there are also some fairly notable differences. They're also rather unpleasant to other fish, either trying to eat them or just taking a bite of them for whatever reason. Though the largest mormyrids are piscivores too, they'll leave similar-sized fish alone if kept in a big enough tank.
