Yellowtail, as in Apolemichthys xanthurus? A couple dwarf species of angels are also some times called yellowtails...
If the Apole. sp., then Ocean Nutrition Angel formula as mentioned, but would recommend a slightly more meaty diet. Chopped up pieces of scallops, fresh live clams that you shuck (sometimes available at grocery stores fish section as "littleneck clams"). H20 foods also sells frozen clams on the half shell if you can't find live. Mysis, specifically Piscine Energetics (PE Mysis) is the best type of mysis to feed. Definitely recommend New Life Spectrum pellets as well. Depending on the size of the fish, don't be afraid to try the larger 3mm pellets instead of the small 1mm. And of course nori sheets. I use the $2.00 ones from Wal-Mart.
A good daily diets is PE Mysis, some NLS, and a nori sheet. Feed the Angel Formula about 3-4x a week. And the meaty foods switching up what you feed, every other day.