We had 6 Panda Corys in our 48 gallon tank. Today we found one had died. While 4 of the others look fine, one seems to have about half of its tail missing. I am not sure the cause of it. Now, we've suddenly just noticed that about 3 of our Pandas have very small portions of there "top" fin missing. Can anyone assist by telling us what might be wrong and what we should do (if anything)?