Remarks: Ruby Tetras are a perfect choice for nano tanks. They are small, shy tetras with a pinkish-red hue and a black tear-drop spot on the caudal peduncle. Because of their size and relative shyness, we recommend at least 15 of these tetras in any tank larger than 10 gallons. For a small tank, such as 5-10 gallons, a group of 6-10 will suffice provided there are no other fish present.