I had 2 crabs and after we put new decorations in the tank they had a spot to climb up and get some air. They were alright for 2 days but then the female died. I looked at them in the morning and she didn't look right. I waved my finger in front of her but she didn't react. So I poked her and she was limp and fell back into the water. She died on top of the plant. Is it possible that she dried out and died because of it?
Nothing in reality makes sense except in the light of magic.
Like said above, fiddler crabs need brackish water and won't survive long in full freshwater. Also, I'm not sure if the one little spot of air on the plant is enough for them. Fiddler crabs spend more time on land than in the water and they need to be able to move around on soft, dry sand.