a Semi large peaceful fish would be a livingstonii, i have two a male and female and they are pretty cool. the male is about 7 inches now and the female around 5 inches, i got them both as juveniles about 2 inches and they are wonderful fish they swim together and keep to themselves, as i understand it the males can get up to 10 inches not very large but large enough and they are real cool looking. my geophagus likes to chase them around, the male livinstonii put a whoopin on him one day because he bit him, but he still chases my stonii's around he can never catch them because they are much to fast for him, looks like they do it as a game na na na na na you cant catch me, lol