A general rule to use is that a tank for an adult ray should be two times as wide as the max disk width and 4 times a long. Therefore, a ray with a max disk width of 12" should be in a tank at least 48" x 24", while a ray with a max with of 24" should be in a tank at least 96" x 48". Like others have said, bigger is always better. A small retic would be ok in your 48 x 18 for a little while. Once the ray is about 8" or 9" across the disk, you'll want to get a bigger tank.
Pat
Amazon Stingrays
