I think u need like a 35/40 for a red for life. They're supposed to get in the 7" range in captivity, and aren't as motionless as the common wolfs. Im no expert though, but seeing the size of the fish as an adult and its activity level i think 35/40 is more what you would need. You could keep it in the 20 as a baby, but i dont know for how long.
Sent from my Galaxy Nexus using MonsterAquariaNetwork App