I use regular chain sword in my 10 and being that the 15 isn't much bigger it might work for you . I don't know if you have seen my tank so here is a pic.
When I think centerpiece plant, I usually think either a Sword or Crypt - a large, singular plant that draws the eye. The reason I don't think a Pygmy Chain Sword work would well is it's small size and ability to send runners all over the place.