the height is fine for anyray really.
rays enjoy deeper tanks sometimes because they like to swim up the sides of the glass, but they dont need it.
i dare say you could keep a ray perfectly happy in 12" deep water with out manny problems.
the extra depth needed is mainly if you are wanting to add mid/top water tank mates, like arrowana ect
i kept a pair of 16" henlei and a 16" leo in an 18" deep tank with a green asian aro and they were perfectly happy.