Actually, praecox rainbowfish are not a brackish species. They may survive in it because rainbowfish tend to be pretty resilient, but they will do much better (live longer, show better color, etc.) in freshwater. They will also do better in a larger tank, like tophat said, they are active and need some room to swim. Who told you that praecox were brackish fish? Someone must have gotten them confused with one of the blue eyes species. In any case, fiddler crabs and praecox aren't compatible, they have totally different water and tank set-up requirements. If you want to keep the crabs, you should get another (hopefully larger) tank to use as a freshwater set-up for the praecox, or take them back to the store you bought them at, or find someone to take them off your hands.