05-19-2013, 07:03 PM
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Speaking as a biologist and not a fiddler-crab keeper, pet shops do lot of things that work, but may work well only on a short-term basis. Example: Some turtles need to be able to dry out periodically (bask), but stores often don't provide a basking area. Why? The turtles simple don't stay in the store long enough for it to be a problem.
We need more information. Is this a freshwater or saltwater tank? You said "tropical", which doesn't tell us much! If you have the traditional salt-water fiddler crab, remember they are under water a couple of times a day as the tide goes in-out.
In your case, the crab will likely be ok 100% under water (physiologically anyway), but may (depending on the species), do better and behave more normally if there is some land. Some crabs burrow, so if you can manage it and this is a crab species that burrows, making a species-only tank could be your best bet. You could silicone in a divider and have a sandy area available. Try the floating island and see what the crab does. I'm sure someone else here can give you more info.