Rays need room.. The wider the tank, the better. A 180 gallon tank is NOT big enough for any ray in my opinion. Motoro rays esp. Motoro rays can get up to 30" round.. It may take some time to get there, but even at a year, a 10" motoro needs more than a 18" wide tank.. Esp if you have two in there.
Would you want to live in an area that was only 8" wideer then you were.
If you plan to get Motoro rays, plan on a wide tank.
Wider is better with ALL rays.. Plus the wider the tank, the better you have a chance for breeding..
ALL JUST MY OPINION OF COURSE