An ornate will need a bigger tank than a 75g, and don't think "the tank is 18" wide so he'll have ages until he gets to 18" and cant turn around". Believe me, you want them in a bigger tank before then. I noticed a big change in the behaviors of my 16"+ ornate when moved from an 80g (48x20x20") to a 160g (60x28x24"), much less stressed, doesn't get panicky and skittish, doesn't swim crazily against the glass, generally more laid back and happy. Plus they'll grow bigger faster in a larger tank. The biggest ornates I've seen have spent their lives in a 3000L (~8x4x3') tank right from the word go, and they're huge, probably all three are close to 20" in less than a couple of years.