I guess the amazon river contains zero gravels huh ? just pure soft and non sharp sand around the river bottom ..... LOL
I think you guys get carried away with the way you want your tank to look with the excuse of having the best substrate for the ray.
Most river bottom contains HUGE driftwood, tree stump, huge sharp rocks, more rocks, many many more rocks, pebbles, more tree stump and etc. NEvertheless, the rays seems to be doing fine and having a blast cruising through each cracks, crevices and opening.
Bare Bottom - I can see the benefit of bare bottom for maintenance purposes and monitoring eating habits. I am too all for it.
Gravel/pebble - Gravel is fine, but you'll have have apower head to blow out each crevices as food tends to get stuck in between.
Sand - Sand is also fine...the only thing about man made sand is that all grains are uniform in size and shape. This tends to creates a "seal" that seals of anerobic gases that can bring down your pH to a very acidic level. Most freshwater sand bed should only be 1" at the most and you need to stir it up every week or so to provides some good disturbance to the sand..
SOrry if I am being a little to pessimist, I think we're all under estimate what fish do in the wild...
stan