Gars are nice, or some type of larger characins. Also Geos work
well as they are not aggro towards the rays. Plus they provide an
extra benefit by sifting and cleaning the substrate, also by turning it over.
i have had good successes with any mid to top water species that is at least half the size of the rays in the same tank. Any smaller and I have noticed them (not the rays) disappearing, usually overnight.
There always tends to be a rather large bulge on one of the rays the next morning....hmmmmm.
So for what it is worth, -FM