Okay, I read the freshwater stingray entry, and the problem seems to be silica's hardness and sharp edges. I'm not sure whether there are silica sands available with rounded edges and fine enough granules, but that should be possible. My (silica) gravel has rounded edges, but it's not fine enough for rays. I'm not sure whether limestone sand is the answer, as mentioned above.
Perhaps, you could just look at the granules under a microscope or a magnifying-glass? If the edges of the sand are eroded, it should be fine, even if it contains silica.