i also believe it is the sharks. same thing happenned to me, at night. brought back the red-tails ( i found them to be very aggressive), and voila... no more dead or bitten fish tails....
It is very possible that the sharks are nocturnal and are exhibiting that behavior at night when no one is there to see. I would qt the sharks and maybe get a zebra danio or something like that to put in with the sharks and see what happens.