you may want to consider putting a mesh cover to prevent the crabs from escaping. They are well capable of scaling the silicone and out of the tank.
As for the ferns, i believe that they are able to be grown out of water. However, you may want to get the info from the gurus here. You may also want to incorporate a water call system by a distributor from either seaview or Hong Yang. The distributor is called"Octopus". ( At least that is what the box says.)
Cheers!!
Bookmarks