while your fish will appreciate the skulls as caves, i like the tank better without them. but if you like them, keep them. the tank is in your house after all.
but if a primate fell into water with crocodilians, they would be eaten, and their bones would be crushed and broken and swallowed too. and that definitely goes for the crocodile skull. a major cause of prereproductive death in crocodiles is cannibalism by larger older crocs.
Eheim Pimp #296
Canon Pimp #67