The issue of could and should are complex.
Stingrays like bottom surface space to swim around and sift through the sand. So they do not need a high tank but one with larger area. They need sandy bottoms for sifting. Now it also depends on which kind of ray you wish to havem as some get bigger than others. I would say a ray tank should measure 5ft long x 3ft deep x 2-3ft high. cheers jk :thumbup:
A rio 300 is no where near big enough to keep rays in, as jk says, you need a large footprint in a tank, not the depth, i'd say 8x3 is an ideal ray tank, certainly not 4 foot