Same thought I had when I first saw them. I used to take them for granted but with some size on them they really are impressive.
Oh, I didn't even realize Ray had them as well. The gentleman I knew who had them is someone I buy Hybrid Cichlids from. He specializes in things like Flowerhorns, High End Parrots, Platinum and Albino Aro's as well as Albino Rays and such.
At one point I would have told you smaller fish as long as they're open water swimmers you'd be good but I've learned the hard way otherwise not too long ago. The Trachy's used to snag fish that would gravitate toward their water strata or those that hid in the lower parts of the tank. However I've seen them with increasing frequency head toward the surface feverishly sniffing and snapping around. Got my Tapajos Pike who was sizeable and no slouch in the power departrment. They make even my larger fish nervous during feedings including their tanks "King", "Boss", "Alpha" a 14 inch Parrot Hybrid whose almost as high as he is long. They come up from underneath nibbling away ready to grab anything they think they can swallow. And if my experience is any indication yours will eventually start day patroling and will seem to know when you approach the tank even if they're hidden so well they can't see you or vice versa. Before I even have a chance to lift the lid my pair come flying out of their hide. Their lumbering, oafish, hyper movements smashing into and rearranging their tanks decor.

I know I was tempted when I saw them. Love that blue grey color and they would make nice additions since they're not overly aggressive or predatory.