wow only 2 weeks? You shouldn't have ANYTHING in that tank! let alone a shark and stingray!!
first off let me defend myself, I have a shark yes, but its in an EGG case and as soon as it hatches is going into my neighbors 120 until he is about 1ft long, then he is going to the aquarium where he will live happily ever after there where I work...
Lets see where to start where to start... first off sharks and stingrays both need LOTS of room on the bottom of a tank to swim, if each of the ONE of them needs a MIN of 180 gallons then you're tank should be at LEAST 300+ to have JUST those two things in there. No other fish will last in a tank with them, weather the are bottom dwellers or not. Your shark is probably stressed to the MAX. have you tested your water? I bet the ammonia is SOOOO HIGH that its causing it to become sick, which equals not eating. Its probaby stressed from lack of space as well. Your room mate is NOT very bright and doesn't have years of experience if you already have those sort of things in a 2 week old tank. It hasn't even started to cycle.
I'm afraid your best bet is to take EVERYTHING back and go get "Salt water aquariums for dummies" and read it... front to back SEVERAL times. Your roommate needs to know hes killing off endangered species (yes, sharks are endangered) because of too much excitement and a lack of knowledge. Sharks need very percise care in order to be maintained in our home aquarium, and only have my 6 years of training on them at the aquarium ahve I attempted to raise one from an egg without their supervision. the reason I can do this is becase I have teh knowledge at the aquarium at my disposal so its not a problem for me. People who just go out and buy sharks are typically new to the hobby in cases like this.
ALso... find a new LFS. those guys are on something to sell you a shark, ray, and an anemone when your tank hasn't even cycled yet.
return everything and learn all you can first before getting anything else. even if they dont give you store credit or a refund save the poor creatures lives and give em back.
conerning your first question, the shark isn't going to eat regardless of what you do. Its stressed and its obvious why. Not enough room to swim, not an established tank, and terrortory probably too. just because you don't care about how much room they have just makes you a mean person who SHOULDN"T be in this hobby. it would be like me saying "Oh live in a 3x4 BOX the rest of your life, i don't care that you can't move much" to you... you wound't like that. they dont like it either. Just because he "sits there" doesnt mean he isn't going to move. Sharks are nocturnal which means they move around and eat during the EVENING/NIGHT. therefore just toss some live shrimp in there and you'll find them gone-that is IF your tank wasn't the problem.
i know that sounds harsh, i'm just an avid shark protector and i hate to see anyone treat animals like that. I'm really a nice person! LOL. I just think you really need to read up on EVERYTHING and then you'll see hwat exactly you guys are doing wrong.
Also, the people here are VERY knowledgeable and anything they say, totally goes!