If it was me, I'd switch the width and height. 30" wide seems much better for a 18" fish. And a quick, skittish fish as well. Just my opinion, good luck.
I have seen a bunch at 16in by various members. The larger are definitely rare. I have seen multiple 18in ones but those were mostly wild caught, and then one member grew one to 20in. He had 8 in a 800g tank with the smallest at 16in and the largest at 20in.