Frozen food like Brine and Mysis shrimp along with spirulina flakes mixed with a high quality flake pretty much round out a good diet for a Tang.
Like mentioned before seaweeds are digested better so they receive more nutrients from it than any greens you feed them from your fridge.
Cellulose in land plants passes through with less absorption in the intestines which they would have to eat tons more of it just to get those tiny little extras from the Vitamins missing from the seaweed.
The Omega Fatty Acids are more beneficial than tiny bit of Vitamin C and K seaweed is missing depending on the species of algae they're eating of course.
So for me it comes down to digestion and I feel seaweed digests better than land plants even though they have to graze just about all day.
Lettuce as a treat pointless unless you like to feed it for ***** and giggles it would take everyday grazing to make a different in supplement everyone arguing.
Then by doing this your taking away the more beneficial option that seaweed offers.
Actually flake food is very nutritionally deficient.
Relying heavily on protein rich foods, such as brine and mysis is also detrimental to the health of herbivores. And, unless the brine shrimp is fortified in some way, it is actually devoid of much nutritional value.
Also being overlooked is that most Acanthuriidae fish have a gizzard like organ to help break down 'harder to digest foods'.
Cellulose being difficult to break down can actually be an aid to fish in captivity, since their inability to graze nonstop limits feeding opportunities, it allows foods eaten after the cellulose rich foods to stay in the digestive tract longer. Cellulose also can help keep digestive tracts clean and healthy.
The freshness should also be taken into consideration, as time and handling have an affect on the vitamins and nutrients. So fresh, but nutritionally less desireable foods may have more value than dried out foods that are stored longer.
Another thing, nori has high nutritional value and is easily digestible, but it is not nutritionally complete. So relying on as a staple is good, but it still needs to be supplemented.