A small SFE might do for a while, but its going to outgrow the tank so be ready to get rid of it in the future. Thats if you actually meant 20 Gallons. People dont speak in metric when it comes to aquariums. I'm canadian and i always use gallons. Get back to us and tell if its 20 Gallons or Litres. You could put a CBS in, mine is pretty aggressive, he attacks the feeding stick i use to feed him. Pretty strong for his size. He's a larger specimen though.