I've had green emerald crabs and they would grab a sleeping fish and kill it.
I think it depends on how hungry they are, but fish that are in reach are fair game to them. I even witnessed the big one grab a fish by the tail so it couldn't get away.
If there's enough food for them, they'll probably leave the fish alone. But everyone has seen the videos of a fish swimming by the crab and it raises its claws at the fish.
I haven't kept them since.
I think it depends on how hungry they are, but fish that are in reach are fair game to them. I even witnessed the big one grab a fish by the tail so it couldn't get away.
If there's enough food for them, they'll probably leave the fish alone. But everyone has seen the videos of a fish swimming by the crab and it raises its claws at the fish.
I haven't kept them since.