SOunds to me like it is probably a bacterial/viral infection, brought in on the cardinals (which are themselves not susceptible to it). Not necessarily all your fish will get it. Just like any infection in any population, some individuals will be more resistant than others, especially if your population includes different species.
My advice would be to keep a good eye on water stats, and keep a healthy water change schedule, to reduce the potential stress on your remaining fish. Then just sit tight and watch the rest of your fish. Your may lose more, hopefully not. I suspect it will run its course, and then you will not see it again. I certainly hope that is the case !
I lost a couple recently in extremely similar circumstances, and with very similar sounding symptoms
