Find a small piece of rock from your tank about 1"-2"... take a small sheet of Nori (3"x3") fold it in half, place the rock length-wise along the Nori, around the middle. Use a rubber band to secure the Nori to the rock. Place it in the tank on the sand or bottom. Hasn't failed me yet. Just remember to remove the floating excess Nori and rock from the tank around the 3hr mark until you learn how much they will consume.
Also, most Tangs & Angels (Med-to-Lrg) typically like to eat about 3 times a day... although some will graze on algae found on rock.