You should now have a tank split into a water and land section. You can decorate both sections with rocks, wood, plastic aquarium ornaments, etc. Use only driftwood in water, as real wood will decompose and rot. Wood may be used in the land portion. Remember to create lots of hiding spots for your crabs to feel secure and molt in, while keeping an open land are for them to explore. In my experience, fiddlers aren’t the best at climbing, so don’t rely on them being able to. Make sure they have an easy way of going from water to land, such as a sand slope or a ramp.They can climb steep inclines when they want to, but tend not to take the effort as often, leading them to get “stuck” on one side.
Sand is a suitable substrate, however coconut fiber or other loamy substrates can promote natural burrowing tendencies. A gradient from sand on the water side to coco fiber may be best. Live plants such as red mangroves may be grown, and will provide a more natural habitat. Mangroves also have the added bonus of removing organics from the water. Crabgrass and other shore grasses found on beaches can be grown as well.