An electric blue jack dempsey is less aggressive, however, they tend to be more expensive though... so we really need to know your tank size, your tank mates, your budget and so on.
Given the fact you were asked twice about tank size and didn't respond, my guess is you know the tank is/was too small. Removing the JD would be the better option, although, if you intend on housing the CK properly I wouldn't imagine their to be aggression issues in the appropriate size tank.
so, the only thing in your tank is this one clown knife?
In my opinion it would have been much easier and made much more sense to get rid of the knife...