Mudskippers are brackish at a SG of around 1.005-1.015 If you do get one try to stay away from Atlantic mudskippers. They fight with each other a lot. I would recommended the Indian Mudskipper because their smaller and can be put in larger groups. I haven't got anything yet but I have been researching them so I can get one soon. You can sometimes put mudskippers with Fiddler crabs but it depends on the kind of mudskipper and the individual personality. Some people can get them to go together great and others mudskippers kill their crabs. Hope whatever you do works.