The bacteria Flexibactor columnaris is still in the tank, in fact it can remain viable in damp mud for a very extended period. It may be a cichlid specific strain, which is why the cat is unaffected. Hybrid cichlids seem be especially susceptible.
After one of my tanks was infected, I turned off filtration and treated the tank with a couple quarts of Hydrogen peroxide (with a few fish in it) I figured they were goners anyway, but a few survived , If you don't rid your tans of it, it will return, sometimes not immediately, but it could always be there, unless you sanitize all..