Well if you have no live rock or inverts, you should just do hyposalinity in the display tank. However, as NigerBang stated, the hippo tang will ABSOLUTELY need a much larger tank very soon if he is indeed in a 29 gallon aqaurium.
Also, how do you intend to keep a tang in such a small tank without live rock for the tang to forage on? Tangs constantly pick at food all day long.