Mr. T/Hamlet/The Doctor has died. T-H-D was the last of the original four fish purchased Thanksgiving weekend. (T-H-D is the fish on the left with the black tail.)
Mr. T must've gotten into one fight too many.
Hamlet, well, everybody does die in the end.
The Doctor must've been too gravely injured to regenerate.
Or maybe the fish died out of confusion -- three names may've been too many for its little tiny brain to handle.