I'm sorry to hear that
Cardinals are very fragile and it's hard to find out what is wring in a tank.
That's a bummer about your cardinals, I would love for my 10gal to look half that good! I've never taken a moss ball apart before to make it grow flat like that, is there any trick to it?
BTW, I still have two of the Malawa shrimp in my 10gal I got from you still small enough I never know if I'm looking at the same one or not! Gonna do another water change this weekend and add some more plants and maybe some rock..