Unfortunately i can't see the pictures
Is it a oncinus or morrowi?
In my experience they prefer a large particle size food, mine particularly likes prawn, shrimp sticks, & tablets.
By their nature they are secretive & like to hide away - venturing out when they are hungry or on other occasions but they must feel secure - this can be accomplished by providing numerous hiding places. I have seen them active but this was in larger tanks with no areas of cover, just gravel but this is unnatural for them I would not advise this.
I know nothing about stingrays but the RTCxTiger shovelnose will get huge & a 75g is too small as he'll grow real quick & when he does the jaguar would be a possible meal - i have known large pims eat a lot less tasty morsels.