No the purple tetra is not a type of emperor tetra, its species name is Hyphessobrycon metae. Is similar shape to the black neon but is purple, it grows to 5cm and is a peaceful schooler ph under 7 and soft water i think the price is around the $7 mark so they are a little dear. The black phantom is a another nice tetra but i'd prefer the red phantom myself, it is another good schooling tetra and around $5 each. As for the diamond tetra I'm not really a fan but its a bigger sized tetra, sorry dont know much about it. What about congo tetras? they are a relly nice fish and are rather big and would cope in with medium sized cichlids. Bleeding hearts show there best colours in peaty waters and subdued lighting, the rosy is the same but lacks the heart shape botch on the body. Mind you though there is many nice tetras out there and everyones taste is different Let me know what you go with