This species attains just over 2 inches, and lives for 10+ years if maintained properly. In a 20g high (= 24-inch length) tank I would not go above 7. No other upper fish, but a couple/few substrate fish are OK. Whipotail Catfish would be ideal, or a group of 5 corys. And lots of plants, including floating. You can read more of this species in our profile, click on the shaded name, Bloodfin Tetra.
This species and others in the genus are actually best in species tanks on their own, since they can all get feisty and fin nip, and a 20g is "small" space to such fish so this trait can be more prevalent. Substrate fish are usually OK with such fish, being at a different level in the water column.
Last edited by Byron; 09-07-2011 at 11:55 AM..