I have to say that I love tangs also, but I have done my research as all new people to the hobby should.. If you go on any site that sells SWF they will give you the recommended size of a tank for any fish. A 45 gal tank is a far cry from a 100 gal which the tang needs. I agree with all of the above, show the fish some mercy and take it back to the LFS and trade it in. When you can afford a 100 gal tank then go get another Hippo tang. I am already second guessing myself on setting up the 55 gal tank I have and going with a bigger tank because I want fish that will just not work in the 55 gal. If I do that I will save the 55 gal and set it up at a later time with fish that will work in it. My suggestion to you is to take the fish back to the store do some more research and rethink the fish that you want to have in the 45 gal tank. There are some beautiful fish that will work in the current 45 that you have.
Just my .02