I've never kept them, but have done quite a bit of research because they are a goal of mine for someday. What I've found is basically, sandy substrate is a must, excellent water quality is a must, and they need meaty foods. blitzen25bm summed things up pretty well. For me personally I would go with a 2ft by 6ft tank minimum, the standdard 90g has a bit small of a footprint, and a lot of un needed height, but if it is what you have it could work for the smaller ones. One of the local LFS's here in columbus has a stingray display tank, and they do have some fish in with them although not many. With a 90 I imagine it would look pretty empty unless you had something swimming around above the rays. Let us know if you get them and how it works out.
Dave