fiddler crabs in fresh water, make brackish

[fiddlerguy]

My 13 year old son had two fiddler crabs in his small tank, but i guess they need brackish water. One died today. Does anyone know if letting them have access to some brackish water a couple of times a week would do the trick? We have a "crab playground" they can go in and take a dip in brackish water without killing off a glowfish and a "mickey-mouse" fish from putting salt in the same tank. Should have known better.
Thanks! Dan