LionFish says......
Well, to be truthful, I would recommend a tank of more than 240 gallons. I usually recommend 300 gallons to be sufficient for them to do well. Although, you can keep them in smaller tanks, it's always better to be safer than sorry. And very true about the sand. Stingrays require a soft substrate so that they will not be hurt. And the remark about triggers is good, too. Seeing as triggers eat cartilage fish and stingrays and sharks are cartilage fish. So, I would recommend nothing lower than a 240 gallon tank so that the stingray has ample space to grow and swim.