Ah, I guess its just one of those things. At least its not happening to Angels you've had for a while. Keep an eye on the remaining three.
With three angels instead of six, you're tank will be okay and you might have room for one or two more panda corys, who would appreciate more of their species. Mess around with the Aqcalculator and try to stay below 130%. I guesstimated your tank dimensions and you should enter your filter. When you bookmark it, it saves what you've entered so you can go back to it.