Most Rays will also out grow a 180g, and you need to measure it. If it's only 18" deep it's either a REALLY tall 180g or a 125g. Either way that limits you a lot more, depth is way more important than height.
Post the measurements and we'll tell you what exactly it is.
The fish's length should never exceed the aquariums depth, or even be more than 75% of the depth in my opinion. So if it is indeed an 18" deep tank then your Aro will last about 8 months in there before it is to large. They will easily grow 1" per month so the fish would be very cramped at that point if it did start at 10".
Post the measurements and we'll tell you what exactly it is.
The fish's length should never exceed the aquariums depth, or even be more than 75% of the depth in my opinion. So if it is indeed an 18" deep tank then your Aro will last about 8 months in there before it is to large. They will easily grow 1" per month so the fish would be very cramped at that point if it did start at 10".