I looked at the rubbermaid stock tanks not long ago when I was trying to find a way to build a cheap holding pond for my males. If your looking at them online the dimensions they list are very misleading, atleast for our purposes. They say they are something like 5'x6', and they are at the top, but they get smaller as you go lower and the floor is only about 48"x42". I personally wouldn't use one for rays much over 12-14".
IMO your better off getting a good pond liner and building yourself a simple 2x4 and plywood frame for it. I built the frame for one in about 6 hours thanks to my new nail gun.