I don't really advocate the whole "I'll just get a bigger tank when it grows" mentality because we all know how often that doesn't work out...BUT for what its worth ornates take a VERY VERY long time to reach 18 in plus. Mine is over three years old and only about 13-14 inches. He reached 10" in a year or so and then his growth slowed to a crawl. So seriously a 55 will be fine for QUITE some time. (fyi, my ornate is in a 55...*gasp*)