[Limi310]

I don't think they would eat your fish, they are generally scavengers, eating algae and particles they find in the sand and mud. They aren't aggressive to other species, but 2 males may fight together. However they do prefer brackish water to saltwater. They are found on ocean beaches, tide pools, brackish water swamps and creeks, etc. They also are air breathers even though they also have gills, so they need a land area where they are able to get out of the water for periods of time. They also like sand or mud substrate to 'burrow'.
Most pet stores sell them as 'Freshwater Crabs' even though they are not. They can live for a peroid of several weeks to months in FW, but usually don't make it long after that.

I bought a bunch once years ago because they were sold as FW and I didn't know any better......none lived longer then a couple months.

And if there is a hole anywhere on the top, they WILL get out. Several times I found mine walking across the living room floor.