I feed nori sheets that are sold at online health food stores. The sheets come in 8X11" sizes. I usually tear them in half. Then I take a half and fold it 4 times before I clip it to the tank. I never removed uneaten parts as there is never anything left after a few hours. What does not get eaten finds its way into the corners of the tank I cant get to. The net effect on the tank is fairly low. Start by feeding half a sheet and see what happens. If you have alot left then cut that in half and see how much is left until you find a good amount to feed daily or every other day.
All of my tank inhabitants eat the nori. Tangs, clowns, dwarf angels, damsels, gobies.