ive always found oscars to have nice personalities, if you want to expand your selction to include central american cichlids the options really grow.. 300 gallons is plenty of room for these fish to grow and unless they are already very large they wont be outgrowing that tank for some time.. if u would consider central american's then a beautiful fish is a red texas. midas or red devils are fun and pretty although quite agressive, but all of these fish should hold theyre own if theyre not overmatched by size. if u havent already try an oscar, they come in many different patterns and colors and theyre very easy to find, and they are found in the amazon with all the rest of your fish..