Rays are not to be treated like our normal every day community type tank. While your logic may apply to these types of environments, it most definately does not apply to the world of rays in the home aquarium.
Then explain what, precisely, is the reason for the mechanical interval for the water changes.
The ray has no genetic need for water changes. What, exactly, is the condition that you are attempting to alter, by changing the water so frequently? If it's not nitrates, then what is it? If it is nitrates, then what is the nitrate level that you are avoiding, and what would keep a deep sand bed from preventing it?
Or do you think the water change is an end, in and of itself? Do H2O molecules age?