both true and i would like to add that they are very hard to keep in the tank. little houdini's they are. i still haven't found mine and i pretty much sealed the tank it was in
Anyway, Redclaws need a tank that is 60-75% land, the rest should be brackish water and deep enough to submerge themselves fully. Preferably more than that, as to provide enough water volume to make keeping it at healthy parameters easier. It has to be filtered.
Just because of that, they cannot be kept in your average fish tank. Even then, crabs are opportunistic omnivores and won't pass up a chance to eat. If they come across a fish sleeping, they will likely consume it.
Crabs are great little critters, but they need a specialized tank.