I don't think that you can keep cats in brackish water cuz the salt is bad for scaless fish. Discus are really nice, but they are very sensitive to water quality I would wait on those. I also like gouramis: Flame, honeycomb, dwarf, opal etc. they have a lot of color, but I have never had one myself so I'm not sure what they're like temperment wise. They are nice looking fish though.