Is that the Ocean Nutrition Brand? I'm not familiar with it (and I'm not a fish nutritionish ) but I took a look at the specs. The analysis of protein, fat, fibre and ash looks fine to me but I'm not keep on some of the ingredients. (those highlighted in bold.
Shrimp, Plankton, Sardine, Soybean, Wheat Flour, Salmon Egg Oil, Lecithin, Spirulina, Fish Oil, Garlic, Calcium Chloride, Potassium Iodide, Ferrous Sulfate, Manganese Sulfate, Magnesium Carbonate, Zinc Sulfate, MPAX [Marine Protein Amino eXtract: Fish Meals, Hydrolysate, and Select Amino Acids (Arginine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Cystine, Phenylalanine, Threonine, Tryptophan, and Valine], Stabilized Vitamin C, Biotin, Cyanocobalamin, Niacin, Riboflavin, Thiamine, Vitamin E, Ethoxyquin (a preservative), Potassium Sorbate, Astaxanthin, Canthaxanthin, Beta Glucan.
That said, if its working then its ok but I'd add some variation with frozen "marine quartet" (a selection of frozen brine, squid, krill etc) and "tetra fresh delica" again they come in several varieties but consist of meaty foods in a vitamin enriched jelly. I've had good results with this. The other food I use on a regular basis is Hikari pellets and Improvit.
The Regal Tang is primarily a carnivore feeding on small invertebrates so if you try to match that diet using as much variety as you can while avoiding non marine food sources you should be ok.