Have you checked your water parameters? How is that sore on its underside? What do you mean you didn't have time to feed it?
Please do not take this the wrong way, but I really really think you need to bring it back to the store...ask them if you can trade for something else....I just don't think the 90 is right for it, and your substrate seems too rough. The Ray should have been burying itself by now, and by what you are saying...something just isn't right...
ARE YOU SURE no chemicals could have gotten into the tank?