I have two 4" tangs, when the tank was new I had a full sheet of nori available every day because there was nothing else for them to eat and they ate the entire sheet every day. As the tank matured, they started picking at the algae and were leaving Nori behind, I cut the Nori into 1/2 sheets and they finished that.
Now 8 months in, I'm down to a 1/4 sheet every day which they almost always finish.
Never leave the Nori in the tank overnight, it really starts to decompose and smell at that point.
I also cut it into strips that prevent large chunks from being ripped off at once and clogging equipment.
In addition, they are fed spirulina brine shrimp, mysis and a few pellets daily once a day.
Hope this helps