Not a tang, but my Foxface will eat a whole box of algae sheets a day, if I let him. I feed frozen in the morning (different mixes like reef plankton, or angel mix, or emerald entree, or omega brine.) At that time, I put 1 sheet (about 10 x 10) in two clips, so there is a half sheet on each side of the tank. mid may, I will put some veggie pellets in the tank that sink, and later that night, I sprinkle in some meaty formula pellets. Some days, I will switch it up... but The only fish who eat the nori and the veggies pellets are the fox and my rock beauty. They can eat and eat... and then eat some more. My tank is good and clean, and the food all gets eaten so I think it's an okay amount.
I don't think it's ever too much food, unless your tank is dirty, or the food isn't getting eaten.