You could start a clown knife in that tank, and upgrade later...I started my clown knife in a 55 gallon and eventually upgraded to a 125 gallon tank when the knife hit 16"...Their bodies are very flexible, but they will need their room eventually....
For what it's worth, my clown knife is 18" now, and he really doesn't utilize his current tank that much. He doesn't swim around all that much, he runs the tank, but he has his own little area that he usually confines to...However, all fish are different and just because mine doesn't utilize a lot of tank space doesn't mean that another one will not....Good luck!
Jude