Apistogramma despite their size have the personality of big Cichlids and are just as territorial. I personally suggest a Solo one or a pair due to the size of the aquarium. When Apistogramma spawn they are extremely protective of the eggs, fry and will beat the other female to death. This is my experience with Triple Red and Orange Flash Apistogramma. I would recommend a small group of Neon Tetra like was mentioned. Also a 20 gallon long would be better if you decided to upgrade.