Yes, ponds or custom tanks are best for rays, as you want a fair amount of square footage for the bottom. A sand substrate is a must (although some keepers have success with bare-bottomed tanks), and a very stable, seasoned tank with a very stable pH is esential. A few rays (such as the reticulated ray that are most often sold as 'teacups') stay relatively small (about 10"-12" disc diameter), but many, like motoros, can exceed 24" in disc size and will eventually need huge aquaria.