I have just established a 55 gal. community tank and a 29 gal. Dempsey tank. I just got a 18 gal high from a friend for free . I want to get someting in between a peaceful community tank and an aggressive Dempsey tank. I wanted some of your guys opinions on some good fish that fit that description. Keep in mind I don't want the fish to be really aggressive or really nice but in between. Also they have to live in the 18 gal high. It's 20" by 10" by 19". Thanks!