I'd chill on the chemicals and instead focus on improving water quality (daily 5% changes for the next 2-3 wks), increase feeding (24hr nori, plus 5-6 feedings per day of high quality flake or pellet for herbs), drop the salinity to 1.017 (not low enough to kill the ick, but low enough to ease resp. stress), remove any tankmates that are bothering the tang, and buy a bigger tank asap (just being a tank that sml can be stressful for tangs).
Of course, you could just return the tang and get something that will do better in yoru tang, like a dwarf angel, or a pair of tank raised clowns.
If you keep the tang and it survives the ick, be ready for some HLLD that will show up in a few months. And, remember never to listen to anything this LFS tells you ever again. It's not a myth that tangs don't do well in sm. tanks. While the experts may argue all day about what is small for a yellow tang, 99% of the people who have kept YTs will agree that a 29gal is small and not a good setup for a beginer. Always make things easy on yourself, don't go looking for a challenge (i.e. lets see if I can jam this fish into a small tank) - newbies that try to break rules spend lots of time and money trying to save sick/dying fish that show poor color and are no fun to watch. Get an easy to keep fish (like TR marron clown) and get it big and fat and impressive to look at. Leave the skinny, pale, sick tangs to the LFS display tanks.