basically look for something that comes from mangrove swamps as many of those amphibians will go into the water fly river turtles are found in brackish habitats, although you need a large tank for one of them they will coexist fine with larger fish. jsut do your research if you find a reptile or amphibean that lives in mangrove swamps or bays or such and it inhabbits the water region there you go. many crocidillians are also brackish water inhabitants. i had a freind keep whites tree frogs in with mudskippers and archers and they did fine (except that they ate the mudskippers). i hope this helps.